diff --git a/backend/app/api/__init__.py b/backend/app/api/__init__.py
index 4a88c1e..b8dc7c4 100644
--- a/backend/app/api/__init__.py
+++ b/backend/app/api/__init__.py
@@ -38,6 +38,7 @@ def all_blueprints() -> list[Blueprint]:
     from .system_activity import system_activity_bp
     from .system_backup import system_backup_bp
     from .tags import tags_bp
+    from .thumbnails import thumbnails_bp
     return [
         api_bp,
         attachments_bp,
@@ -58,6 +59,7 @@ def all_blueprints() -> list[Blueprint]:
         allowlist_bp,
         aliases_bp,
         ml_admin_bp,
+        thumbnails_bp,
         sources_bp,
         platforms_bp,
         posts_bp,
diff --git a/backend/app/api/thumbnails.py b/backend/app/api/thumbnails.py
new file mode 100644
index 0000000..eff1491
--- /dev/null
+++ b/backend/app/api/thumbnails.py
@@ -0,0 +1,13 @@
+"""Thumbnail admin API: backfill trigger."""
+
+from quart import Blueprint, jsonify
+
+thumbnails_bp = Blueprint("thumbnails", __name__, url_prefix="/api/thumbnails")
+
+
+@thumbnails_bp.route("/backfill", methods=["POST"])
+async def trigger_backfill():
+    from ..tasks.thumbnail import backfill_thumbnails
+
+    r = backfill_thumbnails.delay()
+    return jsonify({"celery_task_id": r.id}), 202
diff --git a/tests/test_api_thumbnails.py b/tests/test_api_thumbnails.py
new file mode 100644
index 0000000..b9d07a4
--- /dev/null
+++ b/tests/test_api_thumbnails.py
@@ -0,0 +1,32 @@
+import pytest
+
+from backend.app import create_app
+from backend.app.celery_app import celery
+
+pytestmark = pytest.mark.integration
+
+
+@pytest.fixture(autouse=True)
+def eager():
+    celery.conf.task_always_eager = True
+    yield
+    celery.conf.task_always_eager = False
+
+
+@pytest.fixture
+async def app():
+    return create_app()
+
+
+@pytest.fixture
+async def client(app):
+    async with app.test_client() as c:
+        yield c
+
+
+@pytest.mark.asyncio
+async def test_trigger_thumbnail_backfill(client):
+    r = await client.post("/api/thumbnails/backfill")
+    assert r.status_code == 202
+    body = await r.get_json()
+    assert "celery_task_id" in body