diff --git a/starlette/testclient.py b/starlette/testclient.py
index 8de1e3e31..48afcb673 100644
--- a/starlette/testclient.py
+++ b/starlette/testclient.py
@@ -368,6 +368,7 @@ def __init__(
         backend: str = "asyncio",
         backend_options: typing.Optional[typing.Dict[str, typing.Any]] = None,
         cookies: httpx._client.CookieTypes = None,
+        headers: typing.Dict[str, str] = None,
     ) -> None:
         self.async_backend = _AsyncBackend(
             backend=backend, backend_options=backend_options or {}
@@ -385,10 +386,13 @@ def __init__(
             raise_server_exceptions=raise_server_exceptions,
             root_path=root_path,
         )
+        if headers is None:
+            headers = {}
+        headers.setdefault("user-agent", "testclient")
         super().__init__(
             app=self.app,
             base_url=base_url,
-            headers={"user-agent": "testclient"},
+            headers=headers,
             transport=transport,
             follow_redirects=True,
             cookies=cookies,
diff --git a/tests/test_testclient.py b/tests/test_testclient.py
index 84fcfbafd..a8715ea2d 100644
--- a/tests/test_testclient.py
+++ b/tests/test_testclient.py
@@ -11,6 +11,7 @@
 from starlette.middleware import Middleware
 from starlette.responses import JSONResponse, Response
 from starlette.routing import Route
+from starlette.testclient import TestClient
 from starlette.websockets import WebSocket, WebSocketDisconnect
 
 
@@ -67,6 +68,25 @@ def homepage(request):
     assert response.json() == {"mock": "example"}
 
 
+def test_testclient_headers_behavior():
+    """
+    We should be able to use the test client with user defined headers.
+
+    This is useful if we need to set custom headers for authentication
+    during tests or in development.
+    """
+
+    client = TestClient(mock_service)
+    assert client.headers.get("user-agent") == "testclient"
+
+    client = TestClient(mock_service, headers={"user-agent": "non-default-agent"})
+    assert client.headers.get("user-agent") == "non-default-agent"
+
+    client = TestClient(mock_service, headers={"Authentication": "Bearer 123"})
+    assert client.headers.get("user-agent") == "testclient"
+    assert client.headers.get("Authentication") == "Bearer 123"
+
+
 def test_use_testclient_as_contextmanager(test_client_factory, anyio_backend_name):
     """
     This test asserts a number of properties that are important for an