diff --git a/pydantic_ai_slim/pydantic_ai/providers/google_vertex.py b/pydantic_ai_slim/pydantic_ai/providers/google_vertex.py
index ec436d764..67a70c675 100644
--- a/pydantic_ai_slim/pydantic_ai/providers/google_vertex.py
+++ b/pydantic_ai_slim/pydantic_ai/providers/google_vertex.py
@@ -116,6 +116,8 @@ def __init__(
 class _VertexAIAuth(httpx.Auth):
     """Auth class for Vertex AI API."""
 
+    _refresh_lock: anyio.Lock = anyio.Lock()
+
     credentials: BaseCredentials | ServiceAccountCredentials | None
 
     def __init__(
@@ -169,10 +171,13 @@ async def _get_credentials(self) -> BaseCredentials | ServiceAccountCredentials:
         return creds
 
     async def _refresh_token(self) -> str:  # pragma: no cover
-        assert self.credentials is not None
-        await anyio.to_thread.run_sync(self.credentials.refresh, Request())  # type: ignore[reportUnknownMemberType]
-        assert isinstance(self.credentials.token, str), f'Expected token to be a string, got {self.credentials.token}'  # type: ignore[reportUnknownMemberType]
-        return self.credentials.token
+        async with self._refresh_lock:
+            assert self.credentials is not None
+            await anyio.to_thread.run_sync(self.credentials.refresh, Request())  # type: ignore[reportUnknownMemberType]
+            assert isinstance(self.credentials.token, str), (  # type: ignore[reportUnknownMemberType]
+                f'Expected token to be a string, got {self.credentials.token}'  # type: ignore[reportUnknownMemberType]
+            )
+            return self.credentials.token
 
 
 async def _async_google_auth() -> tuple[BaseCredentials, str | None]: