diff --git a/singer_sdk/streams/core.py b/singer_sdk/streams/core.py
index 739c254d8..51a968313 100644
--- a/singer_sdk/streams/core.py
+++ b/singer_sdk/streams/core.py
@@ -378,7 +378,7 @@ def _parse_datetime(self, value: str) -> datetime.datetime:  # noqa: PLR6301
 
         # Ensure datetime is timezone-aware
         if not result.tzinfo:
-            result = result.astimezone()
+            result = result.replace(tzinfo=datetime.timezone.utc)
 
         return result