diff --git a/google/api_core/future/polling.py b/google/api_core/future/polling.py
index 6466838f..2f80efb5 100644
--- a/google/api_core/future/polling.py
+++ b/google/api_core/future/polling.py
@@ -34,6 +34,7 @@ class _OperationNotComplete(Exception):
     exceptions.TooManyRequests,
     exceptions.InternalServerError,
     exceptions.BadGateway,
+    exceptions.ServiceUnavailable,
 )
 DEFAULT_RETRY = retry.Retry(predicate=RETRY_PREDICATE)