diff --git a/sdks/python/apache_beam/transforms/sideinputs.py b/sdks/python/apache_beam/transforms/sideinputs.py
index 7d72a02f8874..a38e05d66cbe 100644
--- a/sdks/python/apache_beam/transforms/sideinputs.py
+++ b/sdks/python/apache_beam/transforms/sideinputs.py
@@ -60,7 +60,8 @@ def default_window_mapping_fn(
   def map_via_end(source_window: window.BoundedWindow) -> window.BoundedWindow:
     return list(
         target_window_fn.assign(
-            window.WindowFn.AssignContext(source_window.max_timestamp())))[-1]
+            window.WindowFn.AssignContext(
+                source_window.max_timestamp(), window=source_window)))[-1]
 
   return map_via_end
 
diff --git a/sdks/python/apache_beam/transforms/sideinputs_test.py b/sdks/python/apache_beam/transforms/sideinputs_test.py
index 5f3cf761e1eb..9b79b9d1fa8d 100644
--- a/sdks/python/apache_beam/transforms/sideinputs_test.py
+++ b/sdks/python/apache_beam/transforms/sideinputs_test.py
@@ -39,6 +39,7 @@
 from apache_beam.testing.util import equal_to
 from apache_beam.testing.util import equal_to_per_window
 from apache_beam.transforms import Map
+from apache_beam.transforms import sideinputs
 from apache_beam.transforms import trigger
 from apache_beam.transforms import window
 from apache_beam.utils.timestamp import Timestamp
@@ -489,6 +490,40 @@ def process(
     assert_that(results, equal_to([(num_records, expected_fingerprint)]))
     pipeline.run()
 
+  def test_default_window_mapping_fn_source_window(self):
+    """Test that the default window mapping function will propagate the
+    source window when attempting to assign context.
+    """
+    class StringIDWindow(window.BoundedWindow):
+      """A window defined by an arbitrary string ID."""
+      def __init__(self, window_id: str):
+        super().__init__(self._getTimestampFromProto())
+        self.id = window_id
+
+      @staticmethod
+      def _getTimestampFromProto() -> Timestamp:
+        return Timestamp(micros=0)
+
+    class StringIDWindows(window.NonMergingWindowFn):
+      """ A windowing function that assigns each element a window with ID."""
+      def assign(
+          self, assign_context: window.WindowFn.AssignContext
+      ) -> Iterable[window.BoundedWindow]:
+        if assign_context.element is None:
+          assert assign_context.window is not None
+          return [assign_context.window]
+        return [StringIDWindow(str(assign_context.element))]
+
+      def get_window_coder(self):
+        return None
+
+    mapping_fn = sideinputs.default_window_mapping_fn(StringIDWindows())
+    source_window = StringIDWindows().assign(
+        window.WindowFn.AssignContext(Timestamp(10), element='element'))[0]
+    bounded_window = mapping_fn(source_window)
+    assert bounded_window is not None
+    assert bounded_window.id == 'element'
+
 
 if __name__ == '__main__':
   logging.getLogger().setLevel(logging.DEBUG)