diff --git a/google/pubsub_v1/services/subscriber/async_client.py b/google/pubsub_v1/services/subscriber/async_client.py
index 0f77f314b..2801b92a0 100644
--- a/google/pubsub_v1/services/subscriber/async_client.py
+++ b/google/pubsub_v1/services/subscriber/async_client.py
@@ -28,6 +28,7 @@
     Type,
     Union,
 )
+import warnings
 import pkg_resources
 
 import google.api_core.client_options as ClientOptions  # type: ignore
@@ -935,6 +936,12 @@ async def pull(
         if max_messages is not None:
             request.max_messages = max_messages
 
+        if request.return_immediately:
+            warnings.warn(
+                "The return_immediately flag is deprecated and should be set to False.",
+                category=DeprecationWarning,
+            )
+
         # Wrap the RPC method; this adds retry and timeout information,
         # and friendly error handling.
         rpc = gapic_v1.method_async.wrap_method(
diff --git a/google/pubsub_v1/services/subscriber/client.py b/google/pubsub_v1/services/subscriber/client.py
index 67956a53d..e2fbde711 100644
--- a/google/pubsub_v1/services/subscriber/client.py
+++ b/google/pubsub_v1/services/subscriber/client.py
@@ -31,6 +31,7 @@
     Type,
     Union,
 )
+import warnings
 import pkg_resources
 
 from google.api_core import client_options as client_options_lib  # type: ignore
@@ -1124,6 +1125,12 @@ def pull(
             if max_messages is not None:
                 request.max_messages = max_messages
 
+        if request.return_immediately:
+            warnings.warn(
+                "The return_immediately flag is deprecated and should be set to False.",
+                category=DeprecationWarning,
+            )
+
         # Wrap the RPC method; this adds retry and timeout information,
         # and friendly error handling.
         rpc = self._transport._wrapped_methods[self._transport.pull]
diff --git a/synth.py b/synth.py
index 2c0bc5607..c2140f369 100644
--- a/synth.py
+++ b/synth.py
@@ -76,7 +76,7 @@
     flags=re.MULTILINE | re.DOTALL,
 )
 
-if count < 18:
+if count < 15:
     raise Exception("Expected replacements for gRPC channel options not made.")
 
 # If the emulator is used, force an insecure gRPC channel to avoid SSL errors.
@@ -141,6 +141,35 @@
         \g<0>""",
 )
 
+# Emit deprecation warning if return_immediately flag is set with synchronous pull.
+count = s.replace(
+    "google/pubsub_v1/services/subscriber/*client.py",
+    r"import pkg_resources",
+    "import warnings\n\g<0>",
+)
+count = s.replace(
+    "google/pubsub_v1/services/subscriber/*client.py",
+    r"""
+    ([^\n\S]+(?:async\ )?def\ pull\(.*?->\ pubsub\.PullResponse:.*?)
+    ((?P<indent>[^\n\S]+)\#\ Wrap\ the\ RPC\ method)
+    """,
+    textwrap.dedent(
+    """
+    \g<1>
+    \g<indent>if request.return_immediately:
+    \g<indent>    warnings.warn(
+    \g<indent>        "The return_immediately flag is deprecated and should be set to False.",
+    \g<indent>        category=DeprecationWarning,
+    \g<indent>    )
+
+    \g<2>"""
+    ),
+    flags=re.MULTILINE | re.DOTALL | re.VERBOSE,
+)
+
+if count != 2:
+    raise Exception("Too many or too few replacements in pull() methods.")
+
 # Make sure that client library version is present in user agent header.
 s.replace(
     [
diff --git a/tests/unit/pubsub_v1/subscriber/test_subscriber_client.py b/tests/unit/pubsub_v1/subscriber/test_subscriber_client.py
index 6dad4b12a..79bd213e7 100644
--- a/tests/unit/pubsub_v1/subscriber/test_subscriber_client.py
+++ b/tests/unit/pubsub_v1/subscriber/test_subscriber_client.py
@@ -12,9 +12,12 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+import warnings
+
 from google.auth import credentials
 import grpc
 import mock
+import pytest
 
 from google.api_core.gapic_v1.client_info import METRICS_METADATA_KEY
 from google.cloud.pubsub_v1 import subscriber
@@ -217,3 +220,43 @@ def test_streaming_pull_gapic_monkeypatch():
     transport = client.api._transport
     assert hasattr(transport.streaming_pull, "_prefetch_first_result_")
     assert not transport.streaming_pull._prefetch_first_result_
+
+
+def test_sync_pull_warning_if_return_immediately():
+    client = subscriber.Client()
+    subscription_path = "projects/foo/subscriptions/bar"
+
+    with mock.patch.object(
+        client.api._transport, "_wrapped_methods"
+    ), warnings.catch_warnings(record=True) as warned:
+        client.pull(subscription=subscription_path, return_immediately=True)
+
+    # Setting the deprecated return_immediately flag to True should emit a warning.
+    assert len(warned) == 1
+    assert issubclass(warned[0].category, DeprecationWarning)
+    warning_msg = str(warned[0].message)
+    assert "return_immediately" in warning_msg
+    assert "deprecated" in warning_msg
+
+
+@pytest.mark.asyncio
+async def test_sync_pull_warning_if_return_immediately_async():
+    from google.pubsub_v1.services.subscriber.async_client import SubscriberAsyncClient
+
+    client = SubscriberAsyncClient()
+    subscription_path = "projects/foo/subscriptions/bar"
+
+    patcher = mock.patch(
+        "google.pubsub_v1.services.subscriber.async_client.gapic_v1.method_async.wrap_method",
+        new=mock.AsyncMock,
+    )
+
+    with patcher, warnings.catch_warnings(record=True) as warned:
+        await client.pull(subscription=subscription_path, return_immediately=True)
+
+    # Setting the deprecated return_immediately flag to True should emit a warning.
+    assert len(warned) == 1
+    assert issubclass(warned[0].category, DeprecationWarning)
+    warning_msg = str(warned[0].message)
+    assert "return_immediately" in warning_msg
+    assert "deprecated" in warning_msg