diff --git a/pystac_client/item_search.py b/pystac_client/item_search.py
index e8fcc7a5..de9b51c5 100644
--- a/pystac_client/item_search.py
+++ b/pystac_client/item_search.py
@@ -438,7 +438,8 @@ def _format_bbox(value: Optional[BBoxLike]) -> Optional[BBox]:
 
     @staticmethod
     def _to_utc_isoformat(dt: datetime_) -> str:
-        dt = dt.astimezone(timezone.utc)
+        if dt.tzinfo is not None:
+            dt = dt.astimezone(timezone.utc)
         dt = dt.replace(tzinfo=None)
         return f'{dt.isoformat("T")}Z'
 
diff --git a/tests/test_item_search.py b/tests/test_item_search.py
index cca9d97b..a198181e 100644
--- a/tests/test_item_search.py
+++ b/tests/test_item_search.py
@@ -826,3 +826,12 @@ def test_multiple_collections() -> None:
     )
     collections = set(item.collection_id for item in search.items())
     assert collections == {"sentinel-2-l2a", "landsat-c2-l2"}
+
+
+def test_naive_datetime() -> None:
+    search = ItemSearch(
+        url="https://earth-search.aws.element84.com/v1/search",
+        datetime=datetime(2024, 5, 14, 4, 25, 42, tzinfo=None),
+        method="POST",
+    )
+    assert search.get_parameters()["datetime"] == "2024-05-14T04:25:42Z"