diff --git a/starlette/testclient.py b/starlette/testclient.py
index 978f8f506..1037e38cb 100644
--- a/starlette/testclient.py
+++ b/starlette/testclient.py
@@ -12,6 +12,7 @@
 from urllib.parse import unquote, urljoin
 
 import anyio
+import anyio.from_thread
 import httpx
 from anyio.streams.stapled import StapledObjectStream
 
@@ -398,7 +399,9 @@ def _portal_factory(self) -> typing.Generator[anyio.abc.BlockingPortal, None, No
         if self.portal is not None:
             yield self.portal
         else:
-            with anyio.start_blocking_portal(**self.async_backend) as portal:
+            with anyio.from_thread.start_blocking_portal(
+                **self.async_backend
+            ) as portal:
                 yield portal
 
     def _choose_redirect_arg(
@@ -714,7 +717,7 @@ def websocket_connect(
     def __enter__(self) -> "TestClient":
         with contextlib.ExitStack() as stack:
             self.portal = portal = stack.enter_context(
-                anyio.start_blocking_portal(**self.async_backend)
+                anyio.from_thread.start_blocking_portal(**self.async_backend)
             )
 
             @stack.callback