diff --git a/backend/alembic/versions/a318850726ee_add_custom_emoji.py b/backend/alembic/versions/a318850726ee_add_custom_emoji.py
new file mode 100644
index 0000000..c4390fd
--- /dev/null
+++ b/backend/alembic/versions/a318850726ee_add_custom_emoji.py
@@ -0,0 +1,43 @@
+"""add custom emoji
+
+Revision ID: a318850726ee
+Revises: 319c30e24cd9
+Create Date: 2026-08-28 20:34:27.293658
+
+"""
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+
+
+# revision identifiers, used by Alembic.
+revision: str = 'a318850726ee'
+down_revision: Union[str, Sequence[str], None] = '319c30e24cd9'
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    """Upgrade schema."""
+    # ### commands auto generated by Alembic - please adjust! ###
+    op.create_table('custom_emoji',
+    sa.Column('id', sa.Uuid(), nullable=False),
+    sa.Column('shortcode', sa.String(length=30), nullable=False),
+    sa.Column('storage_filename', sa.String(length=64), nullable=False),
+    sa.Column('content_type', sa.String(length=50), nullable=False),
+    sa.Column('uploaded_by', sa.Uuid(), nullable=False),
+    sa.Column('created_at', sa.DateTime(timezone=True), server_default=sa.text('now()'), nullable=False),
+    sa.ForeignKeyConstraint(['uploaded_by'], ['users.id'], ),
+    sa.PrimaryKeyConstraint('id')
+    )
+    op.create_index(op.f('ix_custom_emoji_shortcode'), 'custom_emoji', ['shortcode'], unique=True)
+    # ### end Alembic commands ###
+
+
+def downgrade() -> None:
+    """Downgrade schema."""
+    # ### commands auto generated by Alembic - please adjust! ###
+    op.drop_index(op.f('ix_custom_emoji_shortcode'), table_name='custom_emoji')
+    op.drop_table('custom_emoji')
+    # ### end Alembic commands ###
diff --git a/backend/app/main.py b/backend/app/main.py
index 4c9658a..b41af0e 100644
--- a/backend/app/main.py
+++ b/backend/app/main.py
@@ -15,6 +15,7 @@ from app.routers import (
     admin,
     auth,
     bots,
+    custom_emoji,
     custom_themes,
     health,
     push,
@@ -93,6 +94,7 @@ def create_app() -> FastAPI:
     app.include_router(users.router)
     app.include_router(push.router)
     app.include_router(custom_themes.router)
+    app.include_router(custom_emoji.router)
     app.include_router(uploads.router)
     app.include_router(admin.router)
     app.include_router(bots.router)
diff --git a/backend/app/models/__init__.py b/backend/app/models/__init__.py
index 3587601..0598c8d 100644
--- a/backend/app/models/__init__.py
+++ b/backend/app/models/__init__.py
@@ -1,6 +1,7 @@
 from app.models.admin_audit_log import AdminAuditLog
 from app.models.api_token import ApiToken
 from app.models.base import Base
+from app.models.custom_emoji import CustomEmoji
 from app.models.custom_theme import CustomTheme
 from app.models.event_subscription import EventSubscription
 from app.models.invite import InviteStatus
@@ -46,5 +47,6 @@ __all__ = [
     "WebhookIncoming",
     "EventSubscription",
     "CustomTheme",
+    "CustomEmoji",
     "LinkPreview",
 ]
diff --git a/backend/app/models/custom_emoji.py b/backend/app/models/custom_emoji.py
new file mode 100644
index 0000000..c05936d
--- /dev/null
+++ b/backend/app/models/custom_emoji.py
@@ -0,0 +1,28 @@
+import uuid
+from datetime import datetime
+
+from sqlalchemy import DateTime, ForeignKey, String, func
+from sqlalchemy.orm import Mapped, mapped_column, relationship
+
+from app.models.base import Base
+
+
+class CustomEmoji(Base):
+    __tablename__ = "custom_emoji"
+
+    id: Mapped[uuid.UUID] = mapped_column(primary_key=True, default=uuid.uuid4)
+    # #18: site-wide, not room-scoped -- kept globally unique so a bare
+    # `:shortcode:` in any message/reaction is unambiguous without also
+    # knowing which room it was posted in. 30 chars, not 32 -- the stored
+    # *reference* in MessageReaction.emoji (String(32)) is the shortcode
+    # wrapped in colons, so this is sized to leave room for both without
+    # widening that column.
+    shortcode: Mapped[str] = mapped_column(String(30), unique=True, index=True, nullable=False)
+    storage_filename: Mapped[str] = mapped_column(String(64), nullable=False)
+    content_type: Mapped[str] = mapped_column(String(50), nullable=False)
+    uploaded_by: Mapped[uuid.UUID] = mapped_column(ForeignKey("users.id"), nullable=False)
+    created_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True), server_default=func.now(), nullable=False
+    )
+
+    uploader = relationship("User")
diff --git a/backend/app/routers/custom_emoji.py b/backend/app/routers/custom_emoji.py
new file mode 100644
index 0000000..f581c72
--- /dev/null
+++ b/backend/app/routers/custom_emoji.py
@@ -0,0 +1,131 @@
+import uuid
+
+from fastapi import APIRouter, Depends, File, Form, HTTPException, UploadFile
+from fastapi.responses import FileResponse
+from sqlalchemy.ext.asyncio import AsyncSession
+
+from app.database import get_db
+from app.dependencies import get_current_user
+from app.models import User
+from app.schemas.custom_emoji import CustomEmojiRead
+from app.services.custom_emoji_service import (
+    SHORTCODE_PATTERN,
+    CustomEmojiNotFoundError,
+    DuplicateShortcodeError,
+    InvalidShortcodeError,
+    NotEmojiOwnerError,
+    create_custom_emoji,
+    delete_custom_emoji,
+    get_custom_emoji_by_shortcode,
+    list_custom_emoji,
+)
+from app.services.upload_settings_service import format_mb, get_upload_settings
+from app.storage import (
+    ALLOWED_IMAGE_CONTENT_TYPES,
+    UPLOADS_DIR,
+    InvalidImageError,
+    UploadTooLargeError,
+    process_image,
+    read_capped,
+    save_file,
+)
+
+# Small and square -- these render inline in message text/reaction pills at
+# roughly text size, nowhere near message-image or avatar dimensions.
+CUSTOM_EMOJI_MAX_DIMENSION = 128
+
+router = APIRouter(prefix="/api/custom-emoji", tags=["custom-emoji"])
+
+
+@router.post("", response_model=CustomEmojiRead, status_code=201)
+async def upload_custom_emoji_endpoint(
+    shortcode: str = Form(...),
+    file: UploadFile = File(...),
+    current_user: User = Depends(get_current_user),
+    db: AsyncSession = Depends(get_db),
+):
+    shortcode = shortcode.strip().lower()
+    # Checked here, before any file processing/saving, so the common
+    # rejection cases (bad format, name taken) never leave an orphaned
+    # file on disk -- create_custom_emoji below still re-checks both
+    # (the actual source of truth, and the only thing that closes the
+    # TOCTOU race on the uniqueness check).
+    if not SHORTCODE_PATTERN.match(shortcode):
+        raise HTTPException(
+            status_code=400,
+            detail="Shortcode must be 2-30 characters: lowercase letters, numbers, hyphens, underscores",
+        )
+    if await get_custom_emoji_by_shortcode(db, shortcode) is not None:
+        raise HTTPException(status_code=409, detail="An emoji with that shortcode already exists")
+
+    if file.content_type not in ALLOWED_IMAGE_CONTENT_TYPES:
+        raise HTTPException(status_code=400, detail="Unsupported image type")
+
+    upload_settings = await get_upload_settings(db)
+    try:
+        data = await read_capped(file, cap=upload_settings.max_upload_bytes)
+    except UploadTooLargeError:
+        raise HTTPException(
+            status_code=413,
+            detail=f"Image exceeds {format_mb(upload_settings.max_upload_bytes)} limit",
+        )
+
+    try:
+        data, ext = process_image(
+            data, file.content_type, square=True, max_dimension=CUSTOM_EMOJI_MAX_DIMENSION
+        )
+    except InvalidImageError:
+        raise HTTPException(status_code=400, detail="File is not a valid image")
+
+    storage_filename = save_file(data, ext)
+    try:
+        return await create_custom_emoji(
+            db, current_user.id, shortcode, storage_filename, file.content_type
+        )
+    except (InvalidShortcodeError, DuplicateShortcodeError):
+        # Already checked above -- only reachable via the uniqueness
+        # check's TOCTOU race (two uploads of the same new shortcode at
+        # once), not the common case.
+        raise HTTPException(status_code=409, detail="An emoji with that shortcode already exists")
+
+
+@router.get("", response_model=list[CustomEmojiRead])
+async def list_custom_emoji_endpoint(
+    current_user: User = Depends(get_current_user),
+    db: AsyncSession = Depends(get_db),
+):
+    return await list_custom_emoji(db)
+
+
+@router.delete("/{emoji_id}", status_code=204)
+async def delete_custom_emoji_endpoint(
+    emoji_id: uuid.UUID,
+    current_user: User = Depends(get_current_user),
+    db: AsyncSession = Depends(get_db),
+):
+    try:
+        await delete_custom_emoji(db, emoji_id, current_user)
+    except CustomEmojiNotFoundError:
+        raise HTTPException(status_code=404, detail="Custom emoji not found")
+    except NotEmojiOwnerError:
+        raise HTTPException(status_code=403, detail="Only the uploader or a site admin can remove this")
+
+
+@router.get("/{shortcode}/image")
+async def get_custom_emoji_image_endpoint(
+    shortcode: str,
+    current_user: User = Depends(get_current_user),
+    db: AsyncSession = Depends(get_db),
+):
+    emoji = await get_custom_emoji_by_shortcode(db, shortcode)
+    if emoji is None:
+        raise HTTPException(status_code=404, detail="Custom emoji not found")
+    return FileResponse(
+        UPLOADS_DIR / emoji.storage_filename,
+        media_type=emoji.content_type,
+        # Site-wide and rarely changed, but a shortcode can be deleted and
+        # re-uploaded with different image data -- short-cache like the
+        # avatar endpoint, not `immutable` like content-addressed message
+        # images.
+        headers={"Cache-Control": "private, max-age=300"},
+    )
diff --git a/backend/app/schemas/custom_emoji.py b/backend/app/schemas/custom_emoji.py
new file mode 100644
index 0000000..1bc9ba7
--- /dev/null
+++ b/backend/app/schemas/custom_emoji.py
@@ -0,0 +1,13 @@
+import uuid
+from datetime import datetime
+
+from pydantic import BaseModel, ConfigDict
+
+
+class CustomEmojiRead(BaseModel):
+    model_config = ConfigDict(from_attributes=True)
+
+    id: uuid.UUID
+    shortcode: str
+    uploaded_by: uuid.UUID
+    created_at: datetime
diff --git a/backend/app/services/custom_emoji_service.py b/backend/app/services/custom_emoji_service.py
new file mode 100644
index 0000000..1249005
--- /dev/null
+++ b/backend/app/services/custom_emoji_service.py
@@ -0,0 +1,80 @@
+import re
+import uuid
+
+from sqlalchemy import select
+from sqlalchemy.exc import IntegrityError
+from sqlalchemy.ext.asyncio import AsyncSession
+
+from app.models import CustomEmoji, User
+from app.storage import delete_file
+
+# Deliberately stricter than the built-in Unicode shortcode set's charset
+# (see frontend/src/lib/emojiShortcodes.ts, which also allows '+') -- this
+# is validating a *new name being chosen*, not matching against an
+# existing fixed list, so there's no reason to allow anything a person
+# wouldn't naturally type. Max 30 chars matches CustomEmoji.shortcode's
+# column width exactly (see that model's comment for why).
+SHORTCODE_PATTERN = re.compile(r"^[a-z0-9_-]{2,30}$")
+
+
+class InvalidShortcodeError(Exception):
+    pass
+
+
+class DuplicateShortcodeError(Exception):
+    pass
+
+
+class CustomEmojiNotFoundError(Exception):
+    pass
+
+
+class NotEmojiOwnerError(Exception):
+    pass
+
+
+async def create_custom_emoji(
+    db: AsyncSession,
+    uploaded_by: uuid.UUID,
+    shortcode: str,
+    storage_filename: str,
+    content_type: str,
+) -> CustomEmoji:
+    if not SHORTCODE_PATTERN.match(shortcode):
+        raise InvalidShortcodeError()
+
+    emoji = CustomEmoji(
+        shortcode=shortcode,
+        storage_filename=storage_filename,
+        content_type=content_type,
+        uploaded_by=uploaded_by,
+    )
+    db.add(emoji)
+    try:
+        await db.commit()
+    except IntegrityError as exc:
+        await db.rollback()
+        raise DuplicateShortcodeError() from exc
+    await db.refresh(emoji)
+    return emoji
+
+
+async def list_custom_emoji(db: AsyncSession) -> list[CustomEmoji]:
+    result = await db.execute(select(CustomEmoji).order_by(CustomEmoji.shortcode))
+    return list(result.scalars().all())
+
+
+async def get_custom_emoji_by_shortcode(db: AsyncSession, shortcode: str) -> CustomEmoji | None:
+    result = await db.execute(select(CustomEmoji).where(CustomEmoji.shortcode == shortcode))
+    return result.scalar_one_or_none()
+
+
+async def delete_custom_emoji(db: AsyncSession, emoji_id: uuid.UUID, current_user: User) -> None:
+    emoji = await db.get(CustomEmoji, emoji_id)
+    if emoji is None:
+        raise CustomEmojiNotFoundError()
+    if emoji.uploaded_by != current_user.id and not current_user.is_site_admin:
+        raise NotEmojiOwnerError()
+    delete_file(emoji.storage_filename)
+    await db.delete(emoji)
+    await db.commit()
diff --git a/backend/app/ws/chat.py b/backend/app/ws/chat.py
index 854a3e4..d015b03 100644
--- a/backend/app/ws/chat.py
+++ b/backend/app/ws/chat.py
@@ -311,7 +311,11 @@ async def chat_endpoint(websocket: WebSocket, db: AsyncSession = Depends(get_db)
                         envelope.room_id is None
                         or envelope.message_id is None
                         or not envelope.emoji
-                        or len(envelope.emoji) > 8
+                        # #18: a raw unicode glyph never gets close to this,
+                        # but a custom emoji reaction is stored as its
+                        # literal `:shortcode:` text (see MessageReaction.emoji's
+                        # String(32) column, which this matches exactly).
+                        or len(envelope.emoji) > 32
                     ):
                         await websocket.send_json(
                             {"type": "error", "detail": "room_id, message_id, and emoji required"}
diff --git a/backend/tests/test_custom_emoji.py b/backend/tests/test_custom_emoji.py
new file mode 100644
index 0000000..428b8cd
--- /dev/null
+++ b/backend/tests/test_custom_emoji.py
@@ -0,0 +1,231 @@
+import io
+import uuid
+
+from PIL import Image
+
+from app.models import User
+from tests.conftest import register_and_login
+
+
+def _unique(prefix: str) -> str:
+    return f"{prefix}-{uuid.uuid4().hex[:8]}"
+
+
+def _png_bytes(size: tuple[int, int] = (10, 10)) -> bytes:
+    buf = io.BytesIO()
+    Image.new("RGB", size, color=(255, 0, 0)).save(buf, format="PNG")
+    return buf.getvalue()
+
+
+async def _make_admin(db_session, user_id: str) -> None:
+    user = await db_session.get(User, uuid.UUID(user_id))
+    user.is_site_admin = True
+    await db_session.commit()
+
+
+async def test_upload_custom_emoji_succeeds(client, db_session):
+    await register_and_login(client, db_session, username=_unique("alice"))
+    resp = await client.post(
+        "/api/custom-emoji",
+        data={"shortcode": "party-parrot"},
+        files={"file": ("parrot.png", _png_bytes(), "image/png")},
+    )
+    assert resp.status_code == 201, resp.text
+    body = resp.json()
+    assert body["shortcode"] == "party-parrot"
+    assert "id" in body
+    assert "created_at" in body
+
+
+async def test_upload_normalizes_shortcode_case(client, db_session):
+    await register_and_login(client, db_session, username=_unique("alice"))
+    resp = await client.post(
+        "/api/custom-emoji",
+        data={"shortcode": "  PartyParrot  "},
+        files={"file": ("parrot.png", _png_bytes(), "image/png")},
+    )
+    assert resp.status_code == 201, resp.text
+    assert resp.json()["shortcode"] == "partyparrot"
+
+
+async def test_upload_rejects_invalid_shortcode(client, db_session):
+    await register_and_login(client, db_session, username=_unique("alice"))
+    resp = await client.post(
+        "/api/custom-emoji",
+        data={"shortcode": "a"},  # too short
+        files={"file": ("x.png", _png_bytes(), "image/png")},
+    )
+    assert resp.status_code == 400
+
+
+async def test_upload_rejects_duplicate_shortcode(client, db_session):
+    await register_and_login(client, db_session, username=_unique("alice"))
+    first = await client.post(
+        "/api/custom-emoji",
+        data={"shortcode": "dupe-test"},
+        files={"file": ("a.png", _png_bytes(), "image/png")},
+    )
+    assert first.status_code == 201, first.text
+
+    second = await client.post(
+        "/api/custom-emoji",
+        data={"shortcode": "dupe-test"},
+        files={"file": ("b.png", _png_bytes(), "image/png")},
+    )
+    assert second.status_code == 409
+
+
+async def test_upload_rejects_non_image(client, db_session):
+    await register_and_login(client, db_session, username=_unique("alice"))
+    resp = await client.post(
+        "/api/custom-emoji",
+        data={"shortcode": "not-an-image"},
+        files={"file": ("x.txt", b"hello", "text/plain")},
+    )
+    assert resp.status_code == 400
+
+
+async def test_upload_rejects_oversized(client, db_session):
+    await register_and_login(client, db_session, username=_unique("alice"))
+    oversized = b"0" * (9 * 1024 * 1024)
+    resp = await client.post(
+        "/api/custom-emoji",
+        data={"shortcode": "too-big"},
+        files={"file": ("huge.png", oversized, "image/png")},
+    )
+    assert resp.status_code == 413
+
+
+async def test_list_custom_emoji(client, db_session):
+    await register_and_login(client, db_session, username=_unique("alice"))
+    await client.post(
+        "/api/custom-emoji",
+        data={"shortcode": _unique("listed")},
+        files={"file": ("a.png", _png_bytes(), "image/png")},
+    )
+    resp = await client.get("/api/custom-emoji")
+    assert resp.status_code == 200
+    assert len(resp.json()) >= 1
+
+
+async def test_serve_custom_emoji_image_by_shortcode(client, db_session):
+    await register_and_login(client, db_session, username=_unique("alice"))
+    shortcode = _unique("served")
+    upload = await client.post(
+        "/api/custom-emoji",
+        data={"shortcode": shortcode},
+        files={"file": ("a.png", _png_bytes(), "image/png")},
+    )
+    assert upload.status_code == 201
+
+    resp = await client.get(f"/api/custom-emoji/{shortcode}/image")
+    assert resp.status_code == 200
+    assert resp.headers["content-type"] == "image/png"
+
+
+async def test_serve_unknown_shortcode_404s(client, db_session):
+    await register_and_login(client, db_session, username=_unique("alice"))
+    resp = await client.get("/api/custom-emoji/no-such-emoji/image")
+    assert resp.status_code == 404
+
+
+async def test_uploader_can_delete_own_emoji(client, db_session):
+    await register_and_login(client, db_session, username=_unique("alice"))
+    upload = await client.post(
+        "/api/custom-emoji",
+        data={"shortcode": _unique("deleteme")},
+        files={"file": ("a.png", _png_bytes(), "image/png")},
+    )
+    emoji_id = upload.json()["id"]
+
+    resp = await client.delete(f"/api/custom-emoji/{emoji_id}")
+    assert resp.status_code == 204
+
+    listed = (await client.get("/api/custom-emoji")).json()
+    assert emoji_id not in [e["id"] for e in listed]
+
+
+async def test_non_uploader_non_admin_cannot_delete(client, app, db_session):
+    from httpx import ASGITransport, AsyncClient
+
+    await register_and_login(client, db_session, username=_unique("alice"))
+    upload = await client.post(
+        "/api/custom-emoji",
+        data={"shortcode": _unique("guarded")},
+        files={"file": ("a.png", _png_bytes(), "image/png")},
+    )
+    emoji_id = upload.json()["id"]
+
+    async with AsyncClient(transport=ASGITransport(app=app), base_url="http://test") as bob_client:
+        await register_and_login(bob_client, db_session, username=_unique("bob"))
+        resp = await bob_client.delete(f"/api/custom-emoji/{emoji_id}")
+        assert resp.status_code == 403
+
+    listed = (await client.get("/api/custom-emoji")).json()
+    assert emoji_id in [e["id"] for e in listed]
+
+
+async def test_site_admin_can_delete_others_emoji(client, app, db_session):
+    from httpx import ASGITransport, AsyncClient
+
+    alice = await register_and_login(client, db_session, username=_unique("alice"))
+    upload = await client.post(
+        "/api/custom-emoji",
+        data={"shortcode": _unique("admin-deletable")},
+        files={"file": ("a.png", _png_bytes(), "image/png")},
+    )
+    emoji_id = upload.json()["id"]
+
+    async with AsyncClient(transport=ASGITransport(app=app), base_url="http://test") as admin_client:
+        admin = await register_and_login(admin_client, db_session, username=_unique("admin"))
+        await _make_admin(db_session, admin["id"])
+        resp = await admin_client.delete(f"/api/custom-emoji/{emoji_id}")
+        assert resp.status_code == 204
+
+    assert alice["id"]
+
+
+def _register_ws(ws_client, username: str) -> dict:
+    from app.schemas.user import UserCreate
+    from app.services.auth_service import register_user
+
+    async def _seed():
+        async with ws_client.session_factory() as session:
+            await register_user(
+                session,
+                UserCreate(username=username, email=f"{username}@example.com", password="password123"),
+            )
+
+    ws_client.portal.call(_seed)
+    resp = ws_client.post(
+        "/api/auth/login", json={"username_or_email": username, "password": "password123"}
+    )
+    assert resp.status_code == 200, resp.text
+    return resp.json()
+
+
+def test_reaction_accepts_custom_emoji_shortcode_reference(ws_client_factory):
+    instance = ws_client_factory()
+    _register_ws(instance, _unique("alice"))
+    room = instance.post("/api/rooms", json={"name": _unique("general")}).json()
+
+    with instance.websocket_connect("/ws/chat") as ws:
+        ws.send_json({"type": "join", "room_id": room["id"]})
+        assert ws.receive_json()["type"] == "joined"
+        ws.send_json({"type": "message", "room_id": room["id"], "content": "hello"})
+        message = ws.receive_json()
+
+        # A custom emoji reaction is stored as its literal `:shortcode:`
+        # text (14 chars here) -- well past the old 8-char cap that only
+        # ever needed to fit a raw unicode glyph.
+        ws.send_json(
+            {
+                "type": "reaction",
+                "room_id": room["id"],
+                "message_id": message["id"],
+                "emoji": ":party-parrot:",
+            }
+        )
+        reaction_update = ws.receive_json()
+        assert reaction_update["type"] == "reaction_update"
+        assert reaction_update["reactions"][0]["emoji"] == ":party-parrot:"
diff --git a/frontend/src/App.tsx b/frontend/src/App.tsx
index 6303422..1a66d9c 100644
--- a/frontend/src/App.tsx
+++ b/frontend/src/App.tsx
@@ -1,6 +1,7 @@
 import { Navigate, Route, Routes } from 'react-router-dom'
 import { AuthProvider, useAuth } from './context/AuthContext'
 import { ChatSocketProvider } from './context/ChatSocketContext'
+import { CustomEmojiProvider } from './context/CustomEmojiContext'
 import { AdminRoute } from './components/AdminRoute'
 import { DesktopNotificationBridge } from './components/DesktopNotificationBridge'
 import { ProtectedRoute } from './components/ProtectedRoute'
@@ -67,8 +68,10 @@ function AppRoutes() {
   if (!user) return routes
   return (
     <ChatSocketProvider key={user.id}>
-      <DesktopNotificationBridge />
-      {routes}
+      <CustomEmojiProvider>
+        <DesktopNotificationBridge />
+        {routes}
+      </CustomEmojiProvider>
     </ChatSocketProvider>
   )
 }
diff --git a/frontend/src/api/customEmoji.ts b/frontend/src/api/customEmoji.ts
new file mode 100644
index 0000000..5d0527f
--- /dev/null
+++ b/frontend/src/api/customEmoji.ts
@@ -0,0 +1,47 @@
+import { apiFetch, ApiError, NetworkError } from './client'
+import type { CustomEmoji } from '../types'
+
+export function listCustomEmoji(): Promise<CustomEmoji[]> {
+  return apiFetch<CustomEmoji[]>('/api/custom-emoji')
+}
+
+export function getCustomEmojiUrl(shortcode: string): string {
+  return `/api/custom-emoji/${encodeURIComponent(shortcode)}/image`
+}
+
+export function deleteCustomEmoji(id: string): Promise<void> {
+  return apiFetch<void>(`/api/custom-emoji/${id}`, { method: 'DELETE' })
+}
+
+// Raw fetch, not apiFetch -- same multipart-boundary reason as
+// uploadAvatar/uploadRoomImage (a manually-set Content-Type header would
+// omit the boundary the browser generates for FormData).
+export async function uploadCustomEmoji(shortcode: string, file: File): Promise<CustomEmoji> {
+  const formData = new FormData()
+  formData.append('shortcode', shortcode)
+  formData.append('file', file)
+
+  let response: Response
+  try {
+    response = await fetch('/api/custom-emoji', {
+      method: 'POST',
+      credentials: 'include',
+      body: formData,
+    })
+  } catch {
+    throw new NetworkError()
+  }
+
+  if (!response.ok) {
+    let detail = response.statusText
+    try {
+      const body = await response.json()
+      detail = body.detail ?? detail
+    } catch {
+      // response had no JSON body
+    }
+    throw new ApiError(response.status, detail)
+  }
+
+  return (await response.json()) as CustomEmoji
+}
diff --git a/frontend/src/components/Composer.tsx b/frontend/src/components/Composer.tsx
index 45194fe..bbfbdf4 100644
--- a/frontend/src/components/Composer.tsx
+++ b/frontend/src/components/Composer.tsx
@@ -12,6 +12,7 @@ import { useEscapeKey } from '../hooks/useEscapeKey'
 import { useOnlineStatus } from '../hooks/useOnlineStatus'
 import { uploadRoomFile, uploadRoomImage } from '../api/rooms'
 import { getUploadLimit } from '../api/uploads'
+import { useCustomEmoji } from '../context/CustomEmojiContext'
 import { EMOJI_SHORTCODES, SHORTCODE_BY_GLYPH } from '../lib/emojiShortcodes'
 import { formatFileSize } from '../lib/fileSize'
 import { getRecentEmoji, recordEmojiUsed } from '../lib/recentEmoji'
@@ -135,6 +136,7 @@ export function Composer({ roomId, roomName, isDm, members, rooms, disabled, arc
   const [emojiActiveIndex, setEmojiActiveIndex] = useState(0)
   const [dragActive, setDragActive] = useState(false)
   const [attachMenuOpen, setAttachMenuOpen] = useState(false)
+  const { byShortcode: customEmojiByShortcode } = useCustomEmoji()
   const textareaRef = useRef<HTMLTextAreaElement>(null)
   const fileInputRef = useRef<HTMLInputElement>(null)
   // #29: a separate input with an image/video accept hint, so mobile
@@ -170,20 +172,33 @@ export function Composer({ roomId, roomName, isDm, members, rooms, disabled, arc
     const q = emojiQuery.text.toLowerCase()
     // A bare ":" with nothing typed yet -- suggest recently-used emoji
     // (already capped to 8, see recentEmoji.ts) rather than an arbitrary
-    // slice of the ~950 known shortcodes.
+    // slice of the ~950 known shortcodes. A recent custom-emoji pick is
+    // stored as its literal `:shortcode:` (see recordEmojiUsed's call
+    // sites) -- resolved against the live registry the same way, so a
+    // since-deleted one just doesn't show up here.
     if (!q) {
       return getRecentEmoji()
-        .map((glyph) => {
-          const shortcode = SHORTCODE_BY_GLYPH[glyph]
-          return shortcode ? { shortcode, glyph } : null
+        .map((value) => {
+          const customMatch = /^:([a-z0-9_-]+):$/.exec(value)
+          if (customMatch && customEmojiByShortcode.has(customMatch[1])) {
+            return { shortcode: customMatch[1], glyph: null }
+          }
+          const shortcode = SHORTCODE_BY_GLYPH[value]
+          return shortcode ? { shortcode, glyph: value } : null
         })
         .filter((match): match is EmojiShortcodeMatch => match !== null)
     }
-    return Object.keys(EMOJI_SHORTCODES)
+    // Custom emoji surface first -- a smaller, more specific set, and the
+    // whole reason this app has an upload feature at all is for them to be
+    // reachable as easily as the built-in set.
+    const customMatches: EmojiShortcodeMatch[] = [...customEmojiByShortcode.keys()]
+      .filter((shortcode) => shortcode.startsWith(q))
+      .map((shortcode) => ({ shortcode, glyph: null }))
+    const builtinMatches: EmojiShortcodeMatch[] = Object.keys(EMOJI_SHORTCODES)
       .filter((shortcode) => shortcode.startsWith(q))
-      .slice(0, 8)
       .map((shortcode) => ({ shortcode, glyph: EMOJI_SHORTCODES[shortcode] }))
-  }, [emojiQuery])
+    return [...customMatches, ...builtinMatches].slice(0, 8)
+  }, [emojiQuery, customEmojiByShortcode])
 
   useEffect(() => {
     getUploadLimit()
@@ -248,20 +263,27 @@ export function Composer({ roomId, roomName, isDm, members, rooms, disabled, arc
 
   function selectEmojiShortcode(shortcode: string) {
     const query = emojiQuery
+    if (!query) return
+    // A custom emoji has no unicode glyph to substitute -- its literal
+    // `:shortcode:` text is what actually gets stored/rendered (see
+    // MessageContent.tsx's convertCustomEmojiShortcodes), so that's what
+    // goes in the textarea instead of a glyph.
+    const isCustom = customEmojiByShortcode.has(shortcode)
     const glyph = EMOJI_SHORTCODES[shortcode]
-    if (!query || !glyph) return
+    if (!isCustom && !glyph) return
+    const inserted = isCustom ? `:${shortcode}:` : glyph
     // Matches EmojiPicker's own insertEmoji -- a shortcode-completed emoji
     // counts as "used" the same as one picked from the picker, so it
     // shows up there too next time.
-    recordEmojiUsed(glyph)
+    recordEmojiUsed(inserted)
     const el = textareaRef.current
-    const next = value.slice(0, query.start) + glyph + ' ' + value.slice(query.end)
+    const next = value.slice(0, query.start) + inserted + ' ' + value.slice(query.end)
     setValue(next)
     setEmojiQuery(null)
     requestAnimationFrame(() => {
       if (!el) return
       el.focus()
-      const cursor = query.start + glyph.length + 1 // glyph + trailing space
+      const cursor = query.start + inserted.length + 1 // inserted text + trailing space
       el.setSelectionRange(cursor, cursor)
       autoGrow()
     })
diff --git a/frontend/src/components/ComposerAutocomplete.css b/frontend/src/components/ComposerAutocomplete.css
index 77d552c..ed40266 100644
--- a/frontend/src/components/ComposerAutocomplete.css
+++ b/frontend/src/components/ComposerAutocomplete.css
@@ -45,6 +45,13 @@
   line-height: 1;
 }
 
+.composer-autocomplete-custom-emoji {
+  display: block;
+  width: 1rem;
+  height: 1rem;
+  object-fit: contain;
+}
+
 .composer-autocomplete-secondary {
   font-size: 0.76rem;
   color: var(--ds-muted);
diff --git a/frontend/src/components/CustomEmojiUploadModal.tsx b/frontend/src/components/CustomEmojiUploadModal.tsx
new file mode 100644
index 0000000..d8a3c6f
--- /dev/null
+++ b/frontend/src/components/CustomEmojiUploadModal.tsx
@@ -0,0 +1,108 @@
+import { useState, type ChangeEvent, type FormEvent } from 'react'
+import { uploadCustomEmoji } from '../api/customEmoji'
+import { ApiError } from '../api/client'
+import { EMOJI_SHORTCODES } from '../lib/emojiShortcodes'
+import './Modal.css'
+
+interface CustomEmojiUploadModalProps {
+  onClose: () => void
+  onUploaded: () => void
+}
+
+// Mirrors the shortcode charset the backend actually enforces (see
+// backend/app/services/custom_emoji_service.py's SHORTCODE_PATTERN) --
+// checked here too so a bad name shows up immediately next to the field
+// instead of only after a round trip.
+const SHORTCODE_PATTERN = /^[a-z0-9_-]{2,30}$/
+
+export function CustomEmojiUploadModal({ onClose, onUploaded }: CustomEmojiUploadModalProps) {
+  const [shortcode, setShortcode] = useState('')
+  const [file, setFile] = useState<File | null>(null)
+  const [previewUrl, setPreviewUrl] = useState<string | null>(null)
+  const [error, setError] = useState<string | null>(null)
+  const [submitting, setSubmitting] = useState(false)
+
+  function handleFileSelected(e: ChangeEvent<HTMLInputElement>) {
+    const selected = e.target.files?.[0] ?? null
+    if (previewUrl) URL.revokeObjectURL(previewUrl)
+    setFile(selected)
+    setPreviewUrl(selected ? URL.createObjectURL(selected) : null)
+  }
+
+  function handleClose() {
+    if (previewUrl) URL.revokeObjectURL(previewUrl)
+    onClose()
+  }
+
+  const normalizedShortcode = shortcode.trim().toLowerCase()
+  const shortcodeValid = SHORTCODE_PATTERN.test(normalizedShortcode)
+  // A built-in shortcode always wins when :name: is typed in a message
+  // (see MessageContent.tsx's convertShortcodes, which runs first) -- a
+  // custom emoji uploaded under a colliding name would still upload fine,
+  // but could never actually be *reached* by typing its shortcode. Not a
+  // hard block (site-admin-free upload means no server-side authority to
+  // enforce this against ~950 names), just steered away from here.
+  const collidesWithBuiltin = shortcodeValid && normalizedShortcode in EMOJI_SHORTCODES
+
+  async function handleSubmit(e: FormEvent) {
+    e.preventDefault()
+    if (!file || !shortcodeValid) return
+    setSubmitting(true)
+    setError(null)
+    try {
+      await uploadCustomEmoji(normalizedShortcode, file)
+      onUploaded()
+    } catch (err) {
+      setError(err instanceof ApiError ? err.message : String(err))
+      setSubmitting(false)
+    }
+  }
+
+  return (
+    <div className="modal-scrim" onClick={handleClose}>
+      <div className="modal" onClick={(e) => e.stopPropagation()}>
+        <div className="modal-header">
+          <h2>Add custom emoji</h2>
+          <button type="button" className="modal-close" onClick={handleClose} aria-label="Close">
+            &times;
+          </button>
+        </div>
+        <form onSubmit={handleSubmit}>
+          <div className="modal-field-label">Shortcode</div>
+          <input
+            type="text"
+            value={shortcode}
+            onChange={(e) => setShortcode(e.target.value)}
+            placeholder="party-parrot"
+            autoFocus
+          />
+          {shortcode && !shortcodeValid && (
+            <p className="modal-error">
+              2-30 characters: lowercase letters, numbers, hyphens, underscores
+            </p>
+          )}
+          {collidesWithBuiltin && (
+            <p className="modal-error">
+              :{normalizedShortcode}: is already a built-in emoji -- typing it will always show that
+              one instead of yours
+            </p>
+          )}
+          <div className="modal-field-label">Image</div>
+          <input type="file" accept="image/png,image/jpeg,image/gif,image/webp" onChange={handleFileSelected} />
+          {previewUrl && (
+            <img src={previewUrl} alt="Preview" className="custom-emoji-upload-preview" />
+          )}
+          {error && <p className="modal-error">{error}</p>}
+          <div className="modal-actions">
+            <button type="button" className="btn-secondary" onClick={handleClose}>
+              Cancel
+            </button>
+            <button type="submit" className="btn-primary" disabled={submitting || !file || !shortcodeValid}>
+              {submitting ? 'Uploading…' : 'Add emoji'}
+            </button>
+          </div>
+        </form>
+      </div>
+    </div>
+  )
+}
diff --git a/frontend/src/components/EmojiPicker.css b/frontend/src/components/EmojiPicker.css
index f60f9fe..b872a31 100644
--- a/frontend/src/components/EmojiPicker.css
+++ b/frontend/src/components/EmojiPicker.css
@@ -69,6 +69,26 @@
   padding: 4px 4px 2px;
 }
 
+.emoji-picker-category-label-row {
+  display: flex;
+  align-items: center;
+  justify-content: space-between;
+}
+
+.emoji-picker-add-custom {
+  background: transparent;
+  border: none;
+  color: var(--ds-accent);
+  font-size: 0.7rem;
+  font-weight: 600;
+  cursor: pointer;
+  padding: 2px 4px;
+}
+
+.emoji-picker-add-custom:hover {
+  text-decoration: underline;
+}
+
 .emoji-picker-grid {
   display: grid;
   grid-template-columns: repeat(9, 1fr);
@@ -89,6 +109,32 @@
   background: var(--ds-surface-2);
 }
 
+.emoji-picker-item-custom {
+  position: relative;
+}
+
+.emoji-picker-item-remove {
+  position: absolute;
+  top: -2px;
+  right: -2px;
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  width: 14px;
+  height: 14px;
+  border-radius: 50%;
+  background: var(--ds-danger);
+  color: white;
+  font-size: 0.65rem;
+  line-height: 1;
+  opacity: 0;
+  cursor: pointer;
+}
+
+.emoji-picker-item-custom:hover .emoji-picker-item-remove {
+  opacity: 1;
+}
+
 /* The picker is positioned absolutely relative to its trigger button, which
    can sit close enough to a narrow viewport's edge that the full 320px
    width runs off-screen (e.g. the composer's emoji trigger, near the left
diff --git a/frontend/src/components/EmojiPicker.tsx b/frontend/src/components/EmojiPicker.tsx
index 6ec692b..6e503a4 100644
--- a/frontend/src/components/EmojiPicker.tsx
+++ b/frontend/src/components/EmojiPicker.tsx
@@ -1,8 +1,13 @@
-import { useMemo, useState } from 'react'
+import { useMemo, useState, type MouseEvent } from 'react'
+import { deleteCustomEmoji } from '../api/customEmoji'
+import { useAuth } from '../context/AuthContext'
+import { useCustomEmoji } from '../context/CustomEmojiContext'
 import { useEscapeKey } from '../hooks/useEscapeKey'
 import { ALL_EMOJI, EMOJI_CATEGORIES } from '../lib/emoji'
 import { EMOJI_NAMES } from '../lib/emojiNames'
 import { getRecentEmoji, recordEmojiUsed } from '../lib/recentEmoji'
+import { CustomEmojiUploadModal } from './CustomEmojiUploadModal'
+import { EmojiGlyph } from './MessageContent'
 import './EmojiPicker.css'
 
 interface EmojiPickerProps {
@@ -17,7 +22,17 @@ interface EmojiPickerProps {
 // available viewport space) need this to know how much room to check for.
 export const EMOJI_PICKER_MAX_HEIGHT = 380
 
-function searchEmoji(query: string): string[] {
+// Every emoji this picker deals with -- built-in or custom -- is just a
+// string from here on: a raw unicode glyph, or a custom emoji's literal
+// `:shortcode:` reference (see EmojiGlyph in MessageContent.tsx, which
+// resolves either into the right thing to render). Keeping both kinds in
+// the same list/search/recent machinery means there's exactly one grid
+// rendering path instead of a parallel one for custom emoji.
+function titleFor(value: string): string {
+  return EMOJI_NAMES[value]?.name ?? value
+}
+
+function searchEmoji(query: string, customShortcodes: string[]): string[] {
   const q = query.trim().toLowerCase()
   if (!q) return []
   const seen = new Set<string>()
@@ -32,13 +47,21 @@ function searchEmoji(query: string): string[] {
       results.push(emoji)
     }
   }
+  for (const shortcode of customShortcodes) {
+    if (shortcode.toLowerCase().includes(q)) results.push(`:${shortcode}:`)
+  }
   return results
 }
 
 export function EmojiPicker({ onPick, onClose, placement = 'below', align = 'left' }: EmojiPickerProps) {
   useEscapeKey(onClose)
+  const { user } = useAuth()
+  const { list: customEmoji, refresh: refreshCustomEmoji } = useCustomEmoji()
   const [query, setQuery] = useState('')
-  const searchResults = useMemo(() => searchEmoji(query), [query])
+  const [uploadOpen, setUploadOpen] = useState(false)
+  const [deletingId, setDeletingId] = useState<string | null>(null)
+  const customShortcodes = useMemo(() => customEmoji.map((e) => e.shortcode), [customEmoji])
+  const searchResults = useMemo(() => searchEmoji(query, customShortcodes), [query, customShortcodes])
   const searching = query.trim().length > 0
   // A snapshot taken once when the picker opens, not live-updating as picks
   // happen within this same session -- picking an emoji always closes the
@@ -51,6 +74,18 @@ export function EmojiPicker({ onPick, onClose, placement = 'below', align = 'lef
     onPick(emoji)
   }
 
+  async function handleDeleteCustomEmoji(e: MouseEvent, emojiId: string) {
+    // Delete, not pick -- must never bubble to the button's own onClick.
+    e.stopPropagation()
+    setDeletingId(emojiId)
+    try {
+      await deleteCustomEmoji(emojiId)
+      await refreshCustomEmoji()
+    } finally {
+      setDeletingId(null)
+    }
+  }
+
   return (
     <>
       <div className="emoji-picker-scrim" onClick={onClose} />
@@ -75,10 +110,10 @@ export function EmojiPicker({ onPick, onClose, placement = 'below', align = 'lef
                   type="button"
                   role="menuitem"
                   className="emoji-picker-item"
-                  title={EMOJI_NAMES[emoji]?.name}
+                  title={titleFor(emoji)}
                   onClick={() => pick(emoji)}
                 >
-                  {emoji}
+                  <EmojiGlyph value={emoji} />
                 </button>
               ))}
             </div>
@@ -87,6 +122,48 @@ export function EmojiPicker({ onPick, onClose, placement = 'below', align = 'lef
           )
         ) : (
           <>
+            <div className="emoji-picker-category">
+              <div className="emoji-picker-category-label-row">
+                <div className="emoji-picker-category-label">Custom</div>
+                <button
+                  type="button"
+                  className="emoji-picker-add-custom"
+                  onClick={() => setUploadOpen(true)}
+                >
+                  + Add
+                </button>
+              </div>
+              {customEmoji.length > 0 && (
+                <div className="emoji-picker-grid">
+                  {customEmoji.map((e) => {
+                    const canDelete = user?.id === e.uploaded_by || user?.is_site_admin
+                    return (
+                      <button
+                        key={e.id}
+                        type="button"
+                        role="menuitem"
+                        className="emoji-picker-item emoji-picker-item-custom"
+                        title={`:${e.shortcode}:`}
+                        onClick={() => pick(`:${e.shortcode}:`)}
+                      >
+                        <EmojiGlyph value={`:${e.shortcode}:`} />
+                        {canDelete && (
+                          <span
+                            role="button"
+                            aria-label={`Remove :${e.shortcode}:`}
+                            className="emoji-picker-item-remove"
+                            onClick={(ev) => handleDeleteCustomEmoji(ev, e.id)}
+                            style={deletingId === e.id ? { opacity: 0.5, pointerEvents: 'none' } : undefined}
+                          >
+                            ×
+                          </span>
+                        )}
+                      </button>
+                    )
+                  })}
+                </div>
+              )}
+            </div>
             {recent.length > 0 && (
               <div className="emoji-picker-category">
                 <div className="emoji-picker-category-label">Recently used</div>
@@ -97,10 +174,10 @@ export function EmojiPicker({ onPick, onClose, placement = 'below', align = 'lef
                       type="button"
                       role="menuitem"
                       className="emoji-picker-item"
-                      title={EMOJI_NAMES[emoji]?.name}
+                      title={titleFor(emoji)}
                       onClick={() => pick(emoji)}
                     >
-                      {emoji}
+                      <EmojiGlyph value={emoji} />
                     </button>
                   ))}
                 </div>
@@ -116,10 +193,10 @@ export function EmojiPicker({ onPick, onClose, placement = 'below', align = 'lef
                       type="button"
                       role="menuitem"
                       className="emoji-picker-item"
-                      title={EMOJI_NAMES[emoji]?.name}
+                      title={titleFor(emoji)}
                       onClick={() => pick(emoji)}
                     >
-                      {emoji}
+                      <EmojiGlyph value={emoji} />
                     </button>
                   ))}
                 </div>
@@ -128,6 +205,15 @@ export function EmojiPicker({ onPick, onClose, placement = 'below', align = 'lef
           </>
         )}
       </div>
+      {uploadOpen && (
+        <CustomEmojiUploadModal
+          onClose={() => setUploadOpen(false)}
+          onUploaded={() => {
+            refreshCustomEmoji()
+            setUploadOpen(false)
+          }}
+        />
+      )}
     </>
   )
 }
diff --git a/frontend/src/components/EmojiShortcodeAutocomplete.tsx b/frontend/src/components/EmojiShortcodeAutocomplete.tsx
index 622236a..e763ffd 100644
--- a/frontend/src/components/EmojiShortcodeAutocomplete.tsx
+++ b/frontend/src/components/EmojiShortcodeAutocomplete.tsx
@@ -1,8 +1,11 @@
+import { getCustomEmojiUrl } from '../api/customEmoji'
 import './ComposerAutocomplete.css'
 
 export interface EmojiShortcodeMatch {
   shortcode: string
-  glyph: string
+  // null for a custom emoji -- there's no unicode glyph to show, so the
+  // row renders its uploaded image instead (see getCustomEmojiUrl below).
+  glyph: string | null
 }
 
 interface EmojiShortcodeAutocompleteProps {
@@ -34,7 +37,15 @@ export function EmojiShortcodeAutocomplete({
           onClick={() => onPick(match.shortcode)}
           onMouseEnter={() => onHover(i)}
         >
-          <span className="composer-autocomplete-emoji-glyph">{match.glyph}</span>
+          <span className="composer-autocomplete-emoji-glyph">
+            {match.glyph ?? (
+              <img
+                src={getCustomEmojiUrl(match.shortcode)}
+                alt=""
+                className="composer-autocomplete-custom-emoji"
+              />
+            )}
+          </span>
           <span className="composer-autocomplete-primary">:{match.shortcode}:</span>
         </button>
       ))}
diff --git a/frontend/src/components/MessageContent.css b/frontend/src/components/MessageContent.css
new file mode 100644
index 0000000..2a806fb
--- /dev/null
+++ b/frontend/src/components/MessageContent.css
@@ -0,0 +1,13 @@
+/* #18: em-relative, deliberately -- renders correctly inline in message
+   text, inside a reaction pill, and inside the emoji picker's grid without
+   a separate override per context, since each of those already sets its
+   own font-size and this just tracks it. Kept in this file (imported
+   directly by MessageContent.tsx) rather than MessageList.css so it's
+   loaded wherever MessageContent renders -- FilePreviewModal and HelpPage
+   included, not just the message list. */
+.message-custom-emoji {
+  height: 1.2em;
+  width: 1.2em;
+  object-fit: contain;
+  vertical-align: -0.25em;
+}
diff --git a/frontend/src/components/MessageContent.tsx b/frontend/src/components/MessageContent.tsx
index 2e70d0b..0531401 100644
--- a/frontend/src/components/MessageContent.tsx
+++ b/frontend/src/components/MessageContent.tsx
@@ -1,7 +1,10 @@
 import Markdown from 'markdown-to-jsx'
 import type { ReactNode } from 'react'
 import { Link } from 'react-router-dom'
+import { getCustomEmojiUrl } from '../api/customEmoji'
+import { useCustomEmoji } from '../context/CustomEmojiContext'
 import { EMOJI_SHORTCODES } from '../lib/emojiShortcodes'
+import './MessageContent.css'
 
 interface MessageContentProps {
   content: string
@@ -71,6 +74,17 @@ function MarkdownLink({ href, children }: MarkdownLinkProps) {
   if (href === 'sup:') {
     return <sup>{children}</sup>
   }
+  if (href?.startsWith('emoji:')) {
+    const shortcode = href.slice('emoji:'.length)
+    return (
+      <img
+        src={getCustomEmojiUrl(shortcode)}
+        alt={`:${shortcode}:`}
+        title={`:${shortcode}:`}
+        className="message-custom-emoji"
+      />
+    )
+  }
   return (
     <a href={href} target="_blank" rel="noopener noreferrer">
       {children}
@@ -174,6 +188,68 @@ function extractHeadingIds(text: string): { text: string; headingIds: Map<string
   return { text: nextLines.join('\n'), headingIds }
 }
 
+// #18: a *complete* `:name:` that survived convertShortcodes above (it only
+// replaces names it recognizes, so an unmatched one -- built-in or not --
+// passes through untouched) and matches a shortcode this install actually
+// has a custom emoji for. Turns it into `[​:name:​](emoji:name)`, the same
+// link-trick MarkdownLink's other branches use -- deliberately reusing the
+// exact fence/code-span-skip convention every other converter in this file
+// follows, for the same reason (a pasted `:some_key:` in code shouldn't
+// light up as an emoji any more than an unrelated one should).
+const CUSTOM_EMOJI_PATTERN = /:([a-z0-9_-]+):/g
+
+function convertCustomEmojiShortcodes(text: string, shortcodes: Set<string>): string {
+  if (shortcodes.size === 0) return text
+  const lines = text.split('\n')
+  let inFence = false
+  return lines
+    .map((line) => {
+      if (/^\s*```/.test(line)) {
+        inFence = !inFence
+        return line
+      }
+      if (inFence) return line
+      return line
+        .split(/(`+[^`]*`+)/g)
+        .map((part, i) =>
+          i % 2 === 0
+            ? part.replace(CUSTOM_EMOJI_PATTERN, (match, name) =>
+                shortcodes.has(name) ? `[${match}](emoji:${name})` : match,
+              )
+            : part,
+        )
+        .join('')
+    })
+    .join('\n')
+}
+
+// Reaction pills and the "recently used" emoji row don't go through the
+// markdown pipeline at all -- they render a single stored value directly.
+// A custom emoji's value there is its literal `:shortcode:` (see
+// backend's MessageReaction.emoji); this is the equivalent one-value
+// resolution for those spots, so a deleted-since-reacted-with custom
+// emoji degrades to plain `:shortcode:` text instead of a broken image.
+interface EmojiGlyphProps {
+  value: string
+}
+
+export function EmojiGlyph({ value }: EmojiGlyphProps) {
+  const { byShortcode } = useCustomEmoji()
+  const match = /^:([a-z0-9_-]+):$/.exec(value)
+  const shortcode = match?.[1]
+  if (shortcode && byShortcode.has(shortcode)) {
+    return (
+      <img
+        src={getCustomEmojiUrl(shortcode)}
+        alt={value}
+        title={value}
+        className="message-custom-emoji"
+      />
+    )
+  }
+  return <>{value}</>
+}
+
 const MENTION_PATTERN = /@([a-zA-Z0-9_.-]+)/g
 
 // Turns a validated @username into `[@username](mention:username)` --
@@ -295,8 +371,13 @@ export function preprocessMarkdown(text: string): { text: string; headingIds: Ma
 }
 
 export function MessageContent({ content, memberUsernames, myRooms }: MessageContentProps) {
+  const { byShortcode } = useCustomEmoji()
   const withMentions = memberUsernames ? highlightMentions(content, memberUsernames) : content
   const withRoomRefs = myRooms ? highlightRoomReferences(withMentions, myRooms) : withMentions
-  const { text, headingIds } = preprocessMarkdown(convertShortcodes(withRoomRefs))
+  const withCustomEmoji = convertCustomEmojiShortcodes(
+    convertShortcodes(withRoomRefs),
+    new Set(byShortcode.keys()),
+  )
+  const { text, headingIds } = preprocessMarkdown(withCustomEmoji)
   return <Markdown options={createMarkdownOptions(headingIds)}>{preserveLineBreaks(text)}</Markdown>
 }
diff --git a/frontend/src/components/MessageList.tsx b/frontend/src/components/MessageList.tsx
index 3a8fcdd..3bf058a 100644
--- a/frontend/src/components/MessageList.tsx
+++ b/frontend/src/components/MessageList.tsx
@@ -8,7 +8,7 @@ import { EMOJI_PICKER_MAX_HEIGHT, EmojiPicker } from './EmojiPicker'
 import { FilePreviewModal, getPreviewKind } from './FilePreviewModal'
 import { ImageLightbox } from './ImageLightbox'
 import { LinkPreviewCard } from './LinkPreviewCard'
-import { MessageContent } from './MessageContent'
+import { EmojiGlyph, MessageContent } from './MessageContent'
 import { UserAvatar } from './UserAvatar'
 import { VideoLightbox } from './VideoLightbox'
 import './MessageList.css'
@@ -302,7 +302,9 @@ export function MessageList({
                             title={r.user_ids.map(displayNameForUserId).join(', ')}
                             onClick={() => onReact(msg.id, r.emoji)}
                           >
-                            <span>{r.emoji}</span>
+                            <span>
+                              <EmojiGlyph value={r.emoji} />
+                            </span>
                             <span>{r.count}</span>
                           </button>
                         )
diff --git a/frontend/src/components/Modal.css b/frontend/src/components/Modal.css
index 4466e9b..c7d2cf6 100644
--- a/frontend/src/components/Modal.css
+++ b/frontend/src/components/Modal.css
@@ -429,6 +429,17 @@
   color: var(--ds-muted);
 }
 
+.custom-emoji-upload-preview {
+  display: block;
+  width: 64px;
+  height: 64px;
+  object-fit: contain;
+  margin-top: var(--sp-2);
+  background: var(--ds-surface-2);
+  border-radius: var(--radius);
+  border: 1px solid var(--ds-border);
+}
+
 .modal-list-row-action {
   flex: none;
   background: transparent;
diff --git a/frontend/src/context/CustomEmojiContext.tsx b/frontend/src/context/CustomEmojiContext.tsx
new file mode 100644
index 0000000..ac7858e
--- /dev/null
+++ b/frontend/src/context/CustomEmojiContext.tsx
@@ -0,0 +1,49 @@
+import { createContext, useCallback, useContext, useEffect, useMemo, useState, type ReactNode } from 'react'
+import { listCustomEmoji } from '../api/customEmoji'
+import type { CustomEmoji } from '../types'
+
+interface CustomEmojiContextValue {
+  // Every consumer needs one of two things: "does this shortcode exist"
+  // (MessageContent's :shortcode: -> <img> conversion, keyed by name) or
+  // "the full list to render" (EmojiPicker's Custom category) -- a Map
+  // serves both without a second data structure.
+  byShortcode: Map<string, CustomEmoji>
+  list: CustomEmoji[]
+  // Called after a successful upload/delete so every consumer (picker,
+  // already-rendered messages using a shortcode that didn't exist a
+  // moment ago) picks up the change without a full page reload.
+  refresh: () => Promise<void>
+}
+
+const CustomEmojiContext = createContext<CustomEmojiContextValue | undefined>(undefined)
+
+export function CustomEmojiProvider({ children }: { children: ReactNode }) {
+  const [list, setList] = useState<CustomEmoji[]>([])
+
+  const refresh = useCallback(async () => {
+    try {
+      setList(await listCustomEmoji())
+    } catch {
+      // Non-critical -- the app works fine with an empty/stale custom-emoji
+      // set, same treatment as ProfileModal's custom-themes fetch.
+    }
+  }, [])
+
+  useEffect(() => {
+    refresh()
+  }, [refresh])
+
+  const byShortcode = useMemo(() => new Map(list.map((e) => [e.shortcode, e])), [list])
+
+  return (
+    <CustomEmojiContext.Provider value={{ byShortcode, list, refresh }}>
+      {children}
+    </CustomEmojiContext.Provider>
+  )
+}
+
+export function useCustomEmoji(): CustomEmojiContextValue {
+  const ctx = useContext(CustomEmojiContext)
+  if (!ctx) throw new Error('useCustomEmoji must be used within a CustomEmojiProvider')
+  return ctx
+}
diff --git a/frontend/src/types.ts b/frontend/src/types.ts
index 5020d12..08392ff 100644
--- a/frontend/src/types.ts
+++ b/frontend/src/types.ts
@@ -26,6 +26,14 @@ export interface CustomTheme {
   created_at: string
 }
 
+// #18: site-wide, uploaded by any user -- see backend's app/models/custom_emoji.py.
+export interface CustomEmoji {
+  id: string
+  shortcode: string
+  uploaded_by: string
+  created_at: string
+}
+
 export interface User {
   id: string
   username: string