diff --git a/sdks/python/apache_beam/transforms/core.py b/sdks/python/apache_beam/transforms/core.py
index a6fd3b184d4c..297915023c5c 100644
--- a/sdks/python/apache_beam/transforms/core.py
+++ b/sdks/python/apache_beam/transforms/core.py
@@ -3683,8 +3683,16 @@ def _extract_input_pvalues(self, pvalueish):
     return pvalueish, pvalueish
 
   def expand(self, pcolls):
+    windowing = self.get_windowing(pcolls)
     for pcoll in pcolls:
       self._check_pcollection(pcoll)
+      if pcoll.windowing != windowing:
+        _LOGGER.warning(
+            'All input pcollections must have the same window. Windowing for '
+            'flatten set to %s, windowing of pcoll %s set to %s',
+            windowing,
+            pcoll,
+            pcoll.windowing)
     is_bounded = all(pcoll.is_bounded for pcoll in pcolls)
     return pvalue.PCollection(self.pipeline, is_bounded=is_bounded)
 
diff --git a/sdks/python/apache_beam/transforms/core_test.py b/sdks/python/apache_beam/transforms/core_test.py
index a60974ceb706..4fbeaa2ee97a 100644
--- a/sdks/python/apache_beam/transforms/core_test.py
+++ b/sdks/python/apache_beam/transforms/core_test.py
@@ -24,6 +24,9 @@
 import pytest
 
 import apache_beam as beam
+from apache_beam.testing.util import assert_that
+from apache_beam.testing.util import equal_to
+from apache_beam.transforms.window import FixedWindows
 
 
 class TestDoFn1(beam.DoFn):
@@ -136,6 +139,37 @@ def process(self, element):
       self.assertEqual(p6.is_bounded, False)
 
 
+class FlattenTest(unittest.TestCase):
+  def test_flatten_identical_windows(self):
+    with beam.testing.test_pipeline.TestPipeline() as p:
+      source1 = p | "c1" >> beam.Create(
+          [1, 2, 3, 4, 5]) | "w1" >> beam.WindowInto(FixedWindows(100))
+      source2 = p | "c2" >> beam.Create([6, 7, 8]) | "w2" >> beam.WindowInto(
+          FixedWindows(100))
+      source3 = p | "c3" >> beam.Create([9, 10]) | "w3" >> beam.WindowInto(
+          FixedWindows(100))
+      out = (source1, source2, source3) | "flatten" >> beam.Flatten()
+      assert_that(out, equal_to([1, 2, 3, 4, 5, 6, 7, 8, 9, 10]))
+
+  def test_flatten_no_windows(self):
+    with beam.testing.test_pipeline.TestPipeline() as p:
+      source1 = p | "c1" >> beam.Create([1, 2, 3, 4, 5])
+      source2 = p | "c2" >> beam.Create([6, 7, 8])
+      source3 = p | "c3" >> beam.Create([9, 10])
+      out = (source1, source2, source3) | "flatten" >> beam.Flatten()
+      assert_that(out, equal_to([1, 2, 3, 4, 5, 6, 7, 8, 9, 10]))
+
+  def test_flatten_mismatched_windows(self):
+    with beam.testing.test_pipeline.TestPipeline() as p:
+      source1 = p | "c1" >> beam.Create(
+          [1, 2, 3, 4, 5]) | "w1" >> beam.WindowInto(FixedWindows(25))
+      source2 = p | "c2" >> beam.Create([6, 7, 8]) | "w2" >> beam.WindowInto(
+          FixedWindows(100))
+      source3 = p | "c3" >> beam.Create([9, 10]) | "w3" >> beam.WindowInto(
+          FixedWindows(100))
+      _ = (source1, source2, source3) | "flatten" >> beam.Flatten()
+
+
 if __name__ == '__main__':
   logging.getLogger().setLevel(logging.INFO)
   unittest.main()