diff --git a/sdks/python/apache_beam/transforms/combiners_test.py b/sdks/python/apache_beam/transforms/combiners_test.py
index ba9e21f85567..e9256e5f9f7a 100644
--- a/sdks/python/apache_beam/transforms/combiners_test.py
+++ b/sdks/python/apache_beam/transforms/combiners_test.py
@@ -584,26 +584,68 @@ def has_expected_values(actual):
 
       assert_that(result, has_expected_values)
 
-  def test_combining_with_sliding_windows_and_fanout_raises_error(self):
+  def test_combining_with_sliding_windows_and_fanout(self):
+    # SlidingWindows + fanout now works (fixed in 2.73.0).
+    # Previously this raised ValueError due to WindowInto re-evaluating
+    # window assignments. The fix sets windowing strategy metadata directly
+    # on the PCollection without re-assigning windows, mirroring Java's
+    # setWindowingStrategyInternal().
     options = PipelineOptions()
     options.view_as(StandardOptions).streaming = True
-    with self.assertRaises(ValueError):
-      with TestPipeline(options=options) as p:
-        _ = (
-            p
-            | beam.Create([
-                window.TimestampedValue(0, Timestamp(seconds=1666707510)),
-                window.TimestampedValue(1, Timestamp(seconds=1666707511)),
-                window.TimestampedValue(2, Timestamp(seconds=1666707512)),
-                window.TimestampedValue(3, Timestamp(seconds=1666707513)),
-                window.TimestampedValue(5, Timestamp(seconds=1666707515)),
-                window.TimestampedValue(6, Timestamp(seconds=1666707516)),
-                window.TimestampedValue(7, Timestamp(seconds=1666707517)),
-                window.TimestampedValue(8, Timestamp(seconds=1666707518))
-            ])
-            | beam.WindowInto(window.SlidingWindows(10, 5))
-            | beam.CombineGlobally(beam.combiners.ToListCombineFn()).
-            without_defaults().with_fanout(7))
+    with TestPipeline(options=options) as p:
+
+      def has_expected_values(actual):
+        from hamcrest.core import assert_that as hamcrest_assert
+        from hamcrest.library.collection import only_contains
+        ordered = sorted(actual)
+        hamcrest_assert(
+            ordered,
+            only_contains([0, 1, 2, 3], [0, 1, 2, 3, 5, 6, 7, 8], [5, 6, 7, 8]))
+
+      result = (
+          p
+          | beam.Create([
+              window.TimestampedValue(0, Timestamp(seconds=1666707510)),
+              window.TimestampedValue(1, Timestamp(seconds=1666707511)),
+              window.TimestampedValue(2, Timestamp(seconds=1666707512)),
+              window.TimestampedValue(3, Timestamp(seconds=1666707513)),
+              window.TimestampedValue(5, Timestamp(seconds=1666707515)),
+              window.TimestampedValue(6, Timestamp(seconds=1666707516)),
+              window.TimestampedValue(7, Timestamp(seconds=1666707517)),
+              window.TimestampedValue(8, Timestamp(seconds=1666707518))
+          ])
+          | beam.WindowInto(window.SlidingWindows(10, 5))
+          | beam.CombineGlobally(beam.combiners.ToListCombineFn()).
+          without_defaults().with_fanout(7))
+      assert_that(result, has_expected_values)
+
+  def test_combining_with_session_windows_and_fanout(self):
+    options = PipelineOptions()
+    options.view_as(StandardOptions).streaming = True
+    with TestPipeline(options=options) as p:
+
+      def has_expected_values(actual):
+        from hamcrest.core import assert_that as hamcrest_assert
+        from hamcrest.library.collection import only_contains
+        ordered = sorted(actual)
+        hamcrest_assert(ordered, only_contains([0, 1, 2, 3], [5, 6, 7, 8]))
+
+      result = (
+          p
+          | beam.Create([
+              window.TimestampedValue(0, Timestamp(seconds=1666707510)),
+              window.TimestampedValue(1, Timestamp(seconds=1666707511)),
+              window.TimestampedValue(2, Timestamp(seconds=1666707512)),
+              window.TimestampedValue(3, Timestamp(seconds=1666707513)),
+              window.TimestampedValue(5, Timestamp(seconds=1666707515)),
+              window.TimestampedValue(6, Timestamp(seconds=1666707516)),
+              window.TimestampedValue(7, Timestamp(seconds=1666707517)),
+              window.TimestampedValue(8, Timestamp(seconds=1666707518))
+          ])
+          | beam.WindowInto(window.Sessions(2))
+          | beam.CombineGlobally(beam.combiners.ToListCombineFn()).
+          without_defaults().with_fanout(7))
+      assert_that(result, has_expected_values)
 
   def test_MeanCombineFn_combine(self):
     with TestPipeline() as p:
diff --git a/sdks/python/apache_beam/transforms/core.py b/sdks/python/apache_beam/transforms/core.py
index 6d2552a2a6a1..0aada0191750 100644
--- a/sdks/python/apache_beam/transforms/core.py
+++ b/sdks/python/apache_beam/transforms/core.py
@@ -3271,10 +3271,7 @@ def expand(self, pcoll):
     combine_fn = self._combine_fn
     fanout_fn = self._fanout_fn
 
-    if isinstance(pcoll.windowing.windowfn, SlidingWindows):
-      raise ValueError(
-          'CombinePerKey.with_hot_key_fanout does not yet work properly with '
-          'SlidingWindows. See: https://github.com/apache/beam/issues/20528')
+    use_direct_windowing = isinstance(pcoll.windowing.windowfn, SlidingWindows)
 
     class SplitHotCold(DoFn):
       def start_bundle(self):
@@ -3344,14 +3341,31 @@ def StripNonce(nonce_key_value):
 
     cold, hot = pcoll | ParDo(SplitHotCold()).with_outputs('hot', main='cold')
     cold.element_type = typehints.Any  # No multi-output type hints.
-    precombined_hot = (
-        hot
-        # Avoid double counting that may happen with stacked accumulating mode.
-        | 'WindowIntoDiscarding' >> WindowInto(
-            pcoll.windowing, accumulation_mode=AccumulationMode.DISCARDING)
-        | CombinePerKey(PreCombineFn())
-        | Map(StripNonce)
-        | 'WindowIntoOriginal' >> WindowInto(pcoll.windowing))
+
+    if use_direct_windowing:
+      # For SlidingWindows, swap windowing strategy metadata directly on the
+      # PCollection without re-assigning windows. This mirrors Java's
+      # setWindowingStrategyInternal(). Using WindowInto would call
+      # windowfn.assign() which re-evaluates window assignments from
+      # timestamps — with SlidingWindows, this causes accumulators to leak
+      # into adjacent overlapping windows.
+      if pcoll.windowing.accumulation_mode == AccumulationMode.ACCUMULATING:
+        discarding_windowing = copy.copy(pcoll.windowing)
+        discarding_windowing.accumulation_mode = AccumulationMode.DISCARDING
+        hot._windowing = discarding_windowing
+      precombined_hot = (hot | CombinePerKey(PreCombineFn()) | Map(StripNonce))
+      precombined_hot._windowing = pcoll.windowing
+    else:
+      precombined_hot = (
+          hot
+          # Avoid double counting that may happen with stacked accumulating
+          # mode.
+          | 'WindowIntoDiscarding' >> WindowInto(
+              pcoll.windowing, accumulation_mode=AccumulationMode.DISCARDING)
+          | CombinePerKey(PreCombineFn())
+          | Map(StripNonce)
+          | 'WindowIntoOriginal' >> WindowInto(pcoll.windowing))
+
     return ((cold, precombined_hot)
             | Flatten()
             | CombinePerKey(PostCombineFn()))