diff --git a/src/porchlight/admin/routes.py b/src/porchlight/admin/routes.py
index de20d8a..fe7404f 100644
--- a/src/porchlight/admin/routes.py
+++ b/src/porchlight/admin/routes.py
@@ -11,6 +11,15 @@ from porchlight.validation import GroupListInput, ProfileUpdate, UsernameInput,
 
 router = APIRouter(prefix="/admin", tags=["admin"])
 
+ADMIN_GROUP = "admin"
+
+
+async def _is_last_active_admin(request: Request, user: User) -> bool:
+    """True if removing this user's admin access would leave zero active admins."""
+    if not user.active or ADMIN_GROUP not in user.groups:
+        return False
+    return await request.app.state.user_repo.count_active_admins() <= 1
+
 
 async def _get_admin_user(request: Request) -> User | None:
     """Return the current user if they are an admin, else None."""
@@ -198,6 +207,9 @@ async def update_user_groups(
     if user is None:
         return HTMLResponse("User not found", status_code=404)
 
+    if ADMIN_GROUP not in validated.group_list and await _is_last_active_admin(request, user):
+        return HTMLResponse('<div role="alert">Cannot remove the last active admin</div>')
+
     updated = user.model_copy(update={"groups": validated.group_list})
     await user_repo.update(updated)
     return HTMLResponse('<div role="status">Groups updated</div>')
@@ -241,6 +253,9 @@ async def deactivate_user(request: Request, userid: str) -> Response:
     if user is None:
         return HTMLResponse("User not found", status_code=404)
 
+    if await _is_last_active_admin(request, user):
+        return HTMLResponse('<div role="alert">Cannot deactivate the last active admin</div>')
+
     updated = user.model_copy(update={"active": False})
     await user_repo.update(updated)
     return HTMLResponse(
@@ -350,6 +365,10 @@ async def delete_user(request: Request, userid: str) -> Response:
         return HTMLResponse('<div role="alert">Cannot delete your own account</div>')
 
     user_repo = request.app.state.user_repo
+    target = await user_repo.get_by_userid(userid)
+    if target is not None and await _is_last_active_admin(request, target):
+        return HTMLResponse('<div role="alert">Cannot delete the last active admin</div>')
+
     deleted = await user_repo.delete(userid)
     if not deleted:
         return HTMLResponse("User not found", status_code=404)
diff --git a/src/porchlight/store/protocols.py b/src/porchlight/store/protocols.py
index 16d7c1a..53e7c37 100644
--- a/src/porchlight/store/protocols.py
+++ b/src/porchlight/store/protocols.py
@@ -25,6 +25,8 @@ class UserRepository(Protocol):
 
     async def count_users(self, query: str | None = None) -> int: ...
 
+    async def count_active_admins(self) -> int: ...
+
     async def delete(self, userid: str) -> bool: ...
 
 
diff --git a/src/porchlight/store/sqlite/repositories.py b/src/porchlight/store/sqlite/repositories.py
index 41799d2..f9ebd81 100644
--- a/src/porchlight/store/sqlite/repositories.py
+++ b/src/porchlight/store/sqlite/repositories.py
@@ -163,6 +163,17 @@ class SQLiteUserRepository:
                 row = await cursor.fetchone()
         return row[0] if row else 0
 
+    async def count_active_admins(self) -> int:
+        async with self._db.execute(
+            """
+            SELECT COUNT(*) FROM users u
+            JOIN user_groups g ON g.userid = u.userid
+            WHERE u.active = 1 AND g.group_name = 'admin'
+            """
+        ) as cursor:
+            row = await cursor.fetchone()
+        return row[0] if row else 0
+
     async def delete(self, userid: str) -> bool:
         cursor = await self._db.execute("DELETE FROM users WHERE userid = ?", (userid,))
         await self._db.commit()
diff --git a/tests/test_admin/test_admin_routes.py b/tests/test_admin/test_admin_routes.py
index b2138bf..ae87b93 100644
--- a/tests/test_admin/test_admin_routes.py
+++ b/tests/test_admin/test_admin_routes.py
@@ -463,3 +463,70 @@ async def test_admin_cannot_delete_users_last_credential(client: AsyncClient) ->
     # The last credential must survive, and the admin is told why.
     assert await cred_repo.get_password_by_user(target.userid) is not None
     assert "last credential" in res.text.lower()
+
+
+async def _make_sole_admin_target(client: AsyncClient) -> User:
+    """Create a second admin, then return it as the target. The logged-in
+    admin from _login plus this one means 2 admins; callers deactivate the
+    _login admin first if they need this to be the *last* admin."""
+    return await _create_target_user(client, userid="admin2-01", username="admin2", groups=["admin"])
+
+
+@pytest.mark.asyncio
+async def test_cannot_deactivate_last_active_admin(client: AsyncClient) -> None:
+    await _login(client)  # the only admin
+    app = client._transport.app  # type: ignore[union-attr]
+    admin = await app.state.user_repo.get_by_username("admin")
+
+    token = await get_csrf_token(client)
+    res = await client.post(
+        f"/admin/users/{admin.userid}/deactivate",
+        headers={"X-CSRF-Token": token, "HX-Request": "true"},
+    )
+    still = await app.state.user_repo.get_by_userid(admin.userid)
+    assert still.active is True
+    msg = res.text.lower()
+    assert "last" in msg
+    assert "admin" in msg
+
+
+@pytest.mark.asyncio
+async def test_cannot_remove_admin_group_from_last_admin(client: AsyncClient) -> None:
+    await _login(client)
+    app = client._transport.app  # type: ignore[union-attr]
+    admin = await app.state.user_repo.get_by_username("admin")
+
+    token = await get_csrf_token(client)
+    res = await client.post(
+        f"/admin/users/{admin.userid}/groups",
+        data={"groups": "users"},  # drops "admin"
+        headers={"X-CSRF-Token": token, "HX-Request": "true"},
+    )
+    still = await app.state.user_repo.get_by_userid(admin.userid)
+    assert "admin" in still.groups
+    msg = res.text.lower()
+    assert "last" in msg
+    assert "admin" in msg
+
+
+@pytest.mark.asyncio
+async def test_cannot_delete_last_active_admin(client: AsyncClient) -> None:
+    await _login(client)
+    app = client._transport.app  # type: ignore[union-attr]
+    # A second, separate admin to delete (not self — self-delete is blocked separately).
+    target = await _create_target_user(client, userid="otheradmin-01", username="otheradmin", groups=["admin"])
+    # Deactivate the logged-in admin's peer? Instead make target the only ACTIVE admin
+    # by deactivating the _login admin via repo, so deleting target would orphan admins.
+    login_admin = await app.state.user_repo.get_by_username("admin")
+    await app.state.user_repo.update(login_admin.model_copy(update={"active": False}))
+
+    token = await get_csrf_token(client)
+    res = await client.request(
+        "DELETE",
+        f"/admin/users/{target.userid}",
+        headers={"X-CSRF-Token": token, "HX-Request": "true"},
+    )
+    assert await app.state.user_repo.get_by_userid(target.userid) is not None
+    msg = res.text.lower()
+    assert "last" in msg
+    assert "admin" in msg
diff --git a/tests/test_store/test_sqlite_user_repo.py b/tests/test_store/test_sqlite_user_repo.py
index bc6255e..aa2e555 100644
--- a/tests/test_store/test_sqlite_user_repo.py
+++ b/tests/test_store/test_sqlite_user_repo.py
@@ -237,3 +237,14 @@ async def test_roundtrip_preserves_all_fields(user_repo: SQLiteUserRepository) -
     assert sorted(fetched.groups) == ["admin", "users"]
     assert fetched.created_at == user.created_at
     assert fetched.updated_at == user.updated_at
+
+
+async def test_count_active_admins(user_repo: SQLiteUserRepository) -> None:
+    await user_repo.create(_make_user(userid="a1", username="admin1", groups=["admin", "users"]))
+    await user_repo.create(_make_user(userid="a2", username="admin2", groups=["admin"]))
+    # Inactive admin must not count.
+    await user_repo.create(_make_user(userid="a3", username="admin3", groups=["admin"], active=False))
+    # Non-admin must not count.
+    await user_repo.create(_make_user(userid="u1", username="user1", groups=["users"]))
+
+    assert await user_repo.count_active_admins() == 2