diff --git a/starlette/testclient.py b/starlette/testclient.py
index fb2ad6e35..5b0b855c7 100644
--- a/starlette/testclient.py
+++ b/starlette/testclient.py
@@ -7,6 +7,7 @@
 import math
 import sys
 import typing
+import warnings
 from concurrent.futures import Future
 from types import GeneratorType
 from urllib.parse import unquote, urljoin
@@ -426,6 +427,8 @@ def request(  # type: ignore[override]
         timeout: httpx._types.TimeoutTypes | httpx._client.UseClientDefault = httpx._client.USE_CLIENT_DEFAULT,
         extensions: dict[str, typing.Any] | None = None,
     ) -> httpx.Response:
+        if timeout is not httpx.USE_CLIENT_DEFAULT:
+            warnings.warn("The 'timeout' argument is not supported by 'TestClient'.", DeprecationWarning)
         url = self._merge_url(url)
         return super().request(
             method,
diff --git a/tests/test_testclient.py b/tests/test_testclient.py
index 478dbca46..5d8a4fbf2 100644
--- a/tests/test_testclient.py
+++ b/tests/test_testclient.py
@@ -422,3 +422,12 @@ async def app(scope: Scope, receive: Receive, send: Send) -> None:
     with client.websocket_connect("/hello-world", params={"foo": "bar"}) as websocket:
         data = websocket.receive_bytes()
         assert data == b"/hello-world"
+
+
+@pytest.mark.parametrize("method", ["GET", "POST", "PUT", "PATCH", "DELETE", "OPTIONS", "HEAD"])
+def test_timeout_deprecation(method: str) -> None:
+    with pytest.deprecated_call():
+        client = TestClient(mock_service)
+        client.request(method, "/", timeout=1)
+        method_call = getattr(client, method.lower())
+        method_call("/", timeout=1)