diff --git a/sdks/python/apache_beam/io/flink/flink_streaming_impulse_source.py b/sdks/python/apache_beam/io/flink/flink_streaming_impulse_source.py
index d4a0dfc81fc1..1edf743408a0 100644
--- a/sdks/python/apache_beam/io/flink/flink_streaming_impulse_source.py
+++ b/sdks/python/apache_beam/io/flink/flink_streaming_impulse_source.py
@@ -38,7 +38,7 @@ class FlinkStreamingImpulseSource(PTransform):
   def expand(self, pbegin):
     assert isinstance(pbegin, pvalue.PBegin), (
         'Input to transform must be a PBegin but found %s' % pbegin)
-    return pvalue.PCollection(pbegin.pipeline)
+    return pvalue.PCollection(pbegin.pipeline, is_bounded=False)
 
   def get_windowing(self, inputs):
     return Windowing(GlobalWindows())
diff --git a/sdks/python/apache_beam/io/iobase.py b/sdks/python/apache_beam/io/iobase.py
index 6763c57efa80..605c1bf0984d 100644
--- a/sdks/python/apache_beam/io/iobase.py
+++ b/sdks/python/apache_beam/io/iobase.py
@@ -880,7 +880,8 @@ def split_source(unused_impulse):
                   split.start_position, split.stop_position))))
     else:
       # Treat Read itself as a primitive.
-      return pvalue.PCollection(self.pipeline)
+      return pvalue.PCollection(self.pipeline,
+                                is_bounded=self.source.is_bounded())
 
   def get_windowing(self, unused_inputs):
     return core.Windowing(window.GlobalWindows())
diff --git a/sdks/python/apache_beam/pipeline_test.py b/sdks/python/apache_beam/pipeline_test.py
index 2b62745afefa..d1d9d0dfd791 100644
--- a/sdks/python/apache_beam/pipeline_test.py
+++ b/sdks/python/apache_beam/pipeline_test.py
@@ -90,6 +90,16 @@ def reader(self):
     return FakeSource._Reader(self._vals)
 
 
+class FakeUnboundedSource(NativeSource):
+  """Fake unbounded source. Does not work at runtime"""
+
+  def reader(self):
+    return None
+
+  def is_bounded(self):
+    return False
+
+
 class DoubleParDo(beam.PTransform):
   def expand(self, input):
     return input | 'Inner' >> beam.Map(lambda a: a * 2)
@@ -461,6 +471,56 @@ def process(self, element, counter=DoFn.StateParam(BYTES_STATE)):
     p.run()
     self.assertEqual(pcoll.element_type, str)
 
+  def test_track_pcoll_unbounded(self):
+    pipeline = TestPipeline()
+    pcoll1 = pipeline | 'read' >> Read(FakeUnboundedSource())
+    pcoll2 = pcoll1 | 'do1' >> FlatMap(lambda x: [x + 1])
+    pcoll3 = pcoll2 | 'do2' >> FlatMap(lambda x: [x + 1])
+    self.assertIs(pcoll1.is_bounded, False)
+    self.assertIs(pcoll1.is_bounded, False)
+    self.assertIs(pcoll3.is_bounded, False)
+
+  def test_track_pcoll_bounded(self):
+    pipeline = TestPipeline()
+    pcoll1 = pipeline | 'label1' >> Create([1, 2, 3])
+    pcoll2 = pcoll1 | 'do1' >> FlatMap(lambda x: [x + 1])
+    pcoll3 = pcoll2 | 'do2' >> FlatMap(lambda x: [x + 1])
+    self.assertIs(pcoll1.is_bounded, True)
+    self.assertIs(pcoll2.is_bounded, True)
+    self.assertIs(pcoll3.is_bounded, True)
+
+  def test_track_pcoll_bounded_flatten(self):
+    pipeline = TestPipeline()
+    pcoll1_a = pipeline | 'label_a' >> Create([1, 2, 3])
+    pcoll2_a = pcoll1_a | 'do_a' >> FlatMap(lambda x: [x + 1])
+
+    pcoll1_b = pipeline | 'label_b' >> Create([1, 2, 3])
+    pcoll2_b = pcoll1_b | 'do_b' >> FlatMap(lambda x: [x + 1])
+
+    merged = (pcoll2_a, pcoll2_b) | beam.Flatten()
+
+    self.assertIs(pcoll1_a.is_bounded, True)
+    self.assertIs(pcoll2_a.is_bounded, True)
+    self.assertIs(pcoll1_b.is_bounded, True)
+    self.assertIs(pcoll2_b.is_bounded, True)
+    self.assertIs(merged.is_bounded, True)
+
+  def test_track_pcoll_unbounded_flatten(self):
+    pipeline = TestPipeline()
+    pcoll1_bounded = pipeline | 'label1' >> Create([1, 2, 3])
+    pcoll2_bounded = pcoll1_bounded | 'do1' >> FlatMap(lambda x: [x + 1])
+
+    pcoll1_unbounded = pipeline | 'read' >> Read(FakeUnboundedSource())
+    pcoll2_unbounded = pcoll1_unbounded | 'do2' >> FlatMap(lambda x: [x + 1])
+
+    merged = (pcoll2_bounded, pcoll2_unbounded) | beam.Flatten()
+
+    self.assertIs(pcoll1_bounded.is_bounded, True)
+    self.assertIs(pcoll2_bounded.is_bounded, True)
+    self.assertIs(pcoll1_unbounded.is_bounded, False)
+    self.assertIs(pcoll2_unbounded.is_bounded, False)
+    self.assertIs(merged.is_bounded, False)
+
 
 class DoFnTest(unittest.TestCase):
 
diff --git a/sdks/python/apache_beam/pvalue.py b/sdks/python/apache_beam/pvalue.py
index b60d0cfc5746..7c9d869289a6 100644
--- a/sdks/python/apache_beam/pvalue.py
+++ b/sdks/python/apache_beam/pvalue.py
@@ -63,7 +63,8 @@ class PValue(object):
     (3) Has a value which is meaningful if the transform was executed.
   """
 
-  def __init__(self, pipeline, tag=None, element_type=None, windowing=None):
+  def __init__(self, pipeline, tag=None, element_type=None, windowing=None,
+               is_bounded=True):
     """Initializes a PValue with all arguments hidden behind keyword arguments.
 
     Args:
@@ -78,6 +79,7 @@ def __init__(self, pipeline, tag=None, element_type=None, windowing=None):
     # generating this PValue. The field gets initialized when a transform
     # gets applied.
     self.producer = None
+    self.is_bounded = is_bounded
     if windowing:
       self._windowing = windowing
 
@@ -142,11 +144,21 @@ def __reduce_ex__(self, unused_version):
     # of a closure).
     return _InvalidUnpickledPCollection, ()
 
+  @staticmethod
+  def from_(pcoll):
+    """Create a PCollection, using another PCollection as a starting point.
+
+    Transfers relevant attributes.
+    """
+    return PCollection(pcoll.pipeline, is_bounded=pcoll.is_bounded)
+
   def to_runner_api(self, context):
     return beam_runner_api_pb2.PCollection(
         unique_name=self._unique_name(),
         coder_id=context.coder_id_from_element_type(self.element_type),
-        is_bounded=beam_runner_api_pb2.IsBounded.BOUNDED,
+        is_bounded=beam_runner_api_pb2.IsBounded.BOUNDED
+        if self.is_bounded
+        else beam_runner_api_pb2.IsBounded.UNBOUNDED,
         windowing_strategy_id=context.windowing_strategies.get_id(
             self.windowing))
 
@@ -165,7 +177,8 @@ def from_runner_api(proto, context):
         None,
         element_type=context.element_type_from_coder_id(proto.coder_id),
         windowing=context.windowing_strategies.get_by_id(
-            proto.windowing_strategy_id))
+            proto.windowing_strategy_id),
+        is_bounded=proto.is_bounded == beam_runner_api_pb2.IsBounded.BOUNDED)
 
 
 class _InvalidUnpickledPCollection(object):
diff --git a/sdks/python/apache_beam/runners/dataflow/dataflow_runner.py b/sdks/python/apache_beam/runners/dataflow/dataflow_runner.py
index ddc0ccdb2af7..383b9715e14a 100644
--- a/sdks/python/apache_beam/runners/dataflow/dataflow_runner.py
+++ b/sdks/python/apache_beam/runners/dataflow/dataflow_runner.py
@@ -310,7 +310,8 @@ def visit_transform(self, transform_node):
               new_side_input.pvalue = beam.pvalue.PCollection(
                   pipeline,
                   element_type=typehints.KV[
-                      bytes, side_input.pvalue.element_type])
+                      bytes, side_input.pvalue.element_type],
+                  is_bounded=side_input.pvalue.is_bounded)
               parent = transform_node.parent or pipeline._root_transform()
               map_to_void_key = beam.pipeline.AppliedPTransform(
                   pipeline,
@@ -712,7 +713,7 @@ def apply_GroupByKey(self, transform, pcoll, options):
     coders.registry.verify_deterministic(
         coder.key_coder(), 'GroupByKey operation "%s"' % transform.label)
 
-    return pvalue.PCollection(pcoll.pipeline)
+    return pvalue.PCollection.from_(pcoll)
 
   def run_GroupByKey(self, transform_node, options):
     input_tag = transform_node.inputs[0].tag
@@ -894,7 +895,7 @@ def _pardo_fn_data(transform_node, get_label):
             transform_node.inputs[0].windowing)
 
   def apply_CombineValues(self, transform, pcoll, options):
-    return pvalue.PCollection(pcoll.pipeline)
+    return pvalue.PCollection.from_(pcoll)
 
   def run_CombineValues(self, transform_node, options):
     transform = transform_node.transform
@@ -947,7 +948,7 @@ def run_CombineValues(self, transform_node, options):
   def apply_Read(self, transform, pbegin, options):
     if hasattr(transform.source, 'format'):
       # Consider native Read to be a primitive for dataflow.
-      return beam.pvalue.PCollection(pbegin.pipeline)
+      return beam.pvalue.PCollection.from_(pbegin)
     else:
       debug_options = options.view_as(DebugOptions)
       if (
@@ -958,7 +959,7 @@ def apply_Read(self, transform, pbegin, options):
         return self.apply_PTransform(transform, pbegin, options)
       else:
         # Custom Read is also a primitive for non-FnAPI on dataflow.
-        return beam.pvalue.PCollection(pbegin.pipeline)
+        return beam.pvalue.PCollection.from_(pbegin)
 
   def run_Read(self, transform_node, options):
     transform = transform_node.transform
diff --git a/sdks/python/apache_beam/runners/dataflow/native_io/streaming_create.py b/sdks/python/apache_beam/runners/dataflow/native_io/streaming_create.py
index 980ad24c7962..481209ef90b9 100644
--- a/sdks/python/apache_beam/runners/dataflow/native_io/streaming_create.py
+++ b/sdks/python/apache_beam/runners/dataflow/native_io/streaming_create.py
@@ -61,7 +61,7 @@ class Impulse(PTransform):
     def expand(self, pbegin):
       assert isinstance(pbegin, pvalue.PBegin), (
           'Input to Impulse transform must be a PBegin but found %s' % pbegin)
-      return pvalue.PCollection(pbegin.pipeline)
+      return pvalue.PCollection(pbegin.pipeline, is_bounded=False)
 
     def get_windowing(self, inputs):
       return Windowing(GlobalWindows())
diff --git a/sdks/python/apache_beam/runners/direct/direct_runner.py b/sdks/python/apache_beam/runners/direct/direct_runner.py
index 887b9f085477..7ae16a97497d 100644
--- a/sdks/python/apache_beam/runners/direct/direct_runner.py
+++ b/sdks/python/apache_beam/runners/direct/direct_runner.py
@@ -251,7 +251,7 @@ def get_windowing(self, inputs):
 
   def expand(self, pvalue):
     # This is handled as a native transform.
-    return PCollection(self.pipeline)
+    return PCollection(self.pipeline, is_bounded=self._source.is_bounded())
 
 
 class _DirectWriteToPubSubFn(DoFn):
diff --git a/sdks/python/apache_beam/runners/direct/sdf_direct_runner.py b/sdks/python/apache_beam/runners/direct/sdf_direct_runner.py
index 641863243d6a..307679018a5b 100644
--- a/sdks/python/apache_beam/runners/direct/sdf_direct_runner.py
+++ b/sdks/python/apache_beam/runners/direct/sdf_direct_runner.py
@@ -85,7 +85,7 @@ def __init__(self, process_keyed_elements_transform):
     self.sdf = self._process_keyed_elements_transform.sdf
 
   def expand(self, pcoll):
-    return pvalue.PCollection(pcoll.pipeline)
+    return pvalue.PCollection.from_(pcoll)
 
   def new_process_fn(self, sdf):
     return ProcessFn(
diff --git a/sdks/python/apache_beam/runners/sdf_common.py b/sdks/python/apache_beam/runners/sdf_common.py
index e0573289f3bb..072d3dc74492 100644
--- a/sdks/python/apache_beam/runners/sdf_common.py
+++ b/sdks/python/apache_beam/runners/sdf_common.py
@@ -167,4 +167,4 @@ def __init__(
     self.ptransform_side_inputs = ptransform_side_inputs
 
   def expand(self, pcoll):
-    return pvalue.PCollection(pcoll.pipeline)
+    return pvalue.PCollection.from_(pcoll)
diff --git a/sdks/python/apache_beam/testing/test_stream.py b/sdks/python/apache_beam/testing/test_stream.py
index a5e9574331ec..02a860749c4b 100644
--- a/sdks/python/apache_beam/testing/test_stream.py
+++ b/sdks/python/apache_beam/testing/test_stream.py
@@ -135,7 +135,7 @@ def get_windowing(self, unused_inputs):
   def expand(self, pbegin):
     assert isinstance(pbegin, pvalue.PBegin)
     self.pipeline = pbegin.pipeline
-    return pvalue.PCollection(self.pipeline)
+    return pvalue.PCollection(self.pipeline, is_bounded=False)
 
   def _infer_output_coder(self, input_type=None, input_coder=None):
     return self.coder
diff --git a/sdks/python/apache_beam/transforms/core.py b/sdks/python/apache_beam/transforms/core.py
index 807e8b5d3e6f..f08d0700c420 100644
--- a/sdks/python/apache_beam/transforms/core.py
+++ b/sdks/python/apache_beam/transforms/core.py
@@ -1125,7 +1125,7 @@ def expand(self, pcoll):
             'key types. Consider adding an input type hint for this transform.',
             key_coder, self)
 
-    return pvalue.PCollection(pcoll.pipeline)
+    return pvalue.PCollection.from_(pcoll)
 
   def with_outputs(self, *tags, **main_kw):
     """Returns a tagged tuple allowing access to the outputs of a
@@ -2041,7 +2041,7 @@ def infer_output_type(self, input_type):
 
   def expand(self, pcoll):
     self._check_pcollection(pcoll)
-    return pvalue.PCollection(pcoll.pipeline)
+    return pvalue.PCollection.from_(pcoll)
 
 
 @typehints.with_input_types(typing.Tuple[K, typing.Iterable[V]])
@@ -2055,7 +2055,7 @@ def __init__(self, windowing):
 
   def expand(self, pcoll):
     self._check_pcollection(pcoll)
-    return pvalue.PCollection(pcoll.pipeline)
+    return pvalue.PCollection.from_(pcoll)
 
 
 class _GroupAlsoByWindowDoFn(DoFn):
@@ -2338,7 +2338,8 @@ def _extract_input_pvalues(self, pvalueish):
   def expand(self, pcolls):
     for pcoll in pcolls:
       self._check_pcollection(pcoll)
-    result = pvalue.PCollection(self.pipeline)
+    is_bounded = all(pcoll.is_bounded for pcoll in pcolls)
+    result = pvalue.PCollection(self.pipeline, is_bounded=is_bounded)
     result.element_type = typehints.Union[
         tuple(pcoll.element_type for pcoll in pcolls)]
     return result