diff --git a/fastapi/routing.py b/fastapi/routing.py
index 36acb6b89d..cb977e216d 100644
--- a/fastapi/routing.py
+++ b/fastapi/routing.py
@@ -4,6 +4,7 @@ import functools
 import inspect
 import json
 import types
+import warnings
 from collections.abc import (
     AsyncIterator,
     Awaitable,
@@ -679,6 +680,21 @@ def get_request_handler(
                 if isinstance(raw_response, Response):
                     if raw_response.background is None:
                         raw_response.background = solved_result.background_tasks
+                    elif solved_result.background_tasks is not None:
+                        warnings.warn(
+                            "FastAPI has injected BackgroundTasks via "
+                            "dependency injection, but the endpoint returned "
+                            "a Response that already has a `background` set. "
+                            "The dependency-injected background tasks will be "
+                            "discarded in favor of the response's own "
+                            "`background`. To avoid this warning, either add "
+                            "your tasks to the injected `BackgroundTasks` "
+                            "instance instead of setting `background` on the "
+                            "response, or do not inject `BackgroundTasks` at "
+                            "all.",
+                            UserWarning,
+                            stacklevel=1,
+                        )
                     response = raw_response
                 else:
                     response_args = _build_response_args(
diff --git a/tests/test_warn_background_tasks_overwritten.py b/tests/test_warn_background_tasks_overwritten.py
new file mode 100644
index 0000000000..f1116f14ef
--- /dev/null
+++ b/tests/test_warn_background_tasks_overwritten.py
@@ -0,0 +1,90 @@
+"""Test that a warning is emitted when a returned Response already has a background
+task set and the dependency-injected BackgroundTasks would be silently discarded.
+
+Ref: https://github.com/fastapi/fastapi/issues/11215
+"""
+
+import warnings
+
+from fastapi import BackgroundTasks, FastAPI
+from fastapi.testclient import TestClient
+from starlette.background import BackgroundTask
+from starlette.responses import JSONResponse, Response
+
+
+def test_warn_when_response_background_overwrites_injected_tasks():
+    """When the endpoint returns a Response with its own `background`,
+    and the user also injected `BackgroundTasks`, a UserWarning should
+    be emitted so the silent data loss is visible."""
+    app = FastAPI()
+
+    @app.get("/")
+    async def endpoint(tasks: BackgroundTasks):
+        tasks.add_task(lambda: None)
+        return Response(
+            content="Custom response",
+            background=BackgroundTask(lambda: None),
+        )
+
+    client = TestClient(app, raise_server_exceptions=False)
+
+    with warnings.catch_warnings(record=True) as caught:
+        warnings.simplefilter("always")
+        resp = client.get("/")
+
+    assert resp.status_code == 200
+    bg_warnings = [w for w in caught if "background" in str(w.message).lower()]
+    assert len(bg_warnings) == 1
+    assert issubclass(bg_warnings[0].category, UserWarning)
+
+
+def test_no_warn_when_response_has_no_background():
+    """When the endpoint returns a Response without a `background`,
+    no warning should be emitted (the injected tasks are attached)."""
+    app = FastAPI()
+    executed = []
+
+    def bg_task():
+        executed.append(True)
+
+    @app.get("/")
+    async def endpoint(tasks: BackgroundTasks):
+        tasks.add_task(bg_task)
+        return JSONResponse(content={"ok": True})
+
+    client = TestClient(app)
+
+    with warnings.catch_warnings(record=True) as caught:
+        warnings.simplefilter("always")
+        resp = client.get("/")
+
+    assert resp.status_code == 200
+    bg_warnings = [w for w in caught if "background" in str(w.message).lower()]
+    assert len(bg_warnings) == 0
+    # The injected task should have run
+    assert executed == [True]
+
+
+def test_no_warn_when_no_injected_background_tasks():
+    """When the endpoint returns a Response with a `background` but did
+    NOT inject BackgroundTasks, no warning should be emitted."""
+    app = FastAPI()
+    executed = []
+
+    @app.get("/")
+    async def endpoint():
+        return Response(
+            content="ok",
+            background=BackgroundTask(lambda: executed.append(True)),
+        )
+
+    client = TestClient(app)
+
+    with warnings.catch_warnings(record=True) as caught:
+        warnings.simplefilter("always")
+        resp = client.get("/")
+
+    assert resp.status_code == 200
+    bg_warnings = [w for w in caught if "background" in str(w.message).lower()]
+    assert len(bg_warnings) == 0
+    assert executed == [True]