diff --git a/starlette/testclient.py b/starlette/testclient.py
index 63d879098..55973b735 100644
--- a/starlette/testclient.py
+++ b/starlette/testclient.py
@@ -418,6 +418,19 @@ def __init__(
             cookies=cookies,
         )
 
+    @property
+    def auth(self) -> httpx.Auth | None:
+        """
+        Authentication class used when none is passed at the request-level.
+
+        See also [HTTPX Authentication](https://www.python-httpx.org/quickstart/#authentication).
+        """
+        return self._auth
+
+    @auth.setter
+    def auth(self, auth: httpx._types.AuthTypes) -> None:
+        self._auth = self._build_auth(auth)
+
     @contextlib.contextmanager
     def _portal_factory(self) -> Generator[anyio.abc.BlockingPortal, None, None]:
         if self.portal is not None: