diff --git a/sdk/cosmos/azure-cosmos/CHANGELOG.md b/sdk/cosmos/azure-cosmos/CHANGELOG.md
index 011365c27ffc..56a574dca1ad 100644
--- a/sdk/cosmos/azure-cosmos/CHANGELOG.md
+++ b/sdk/cosmos/azure-cosmos/CHANGELOG.md
@@ -10,6 +10,7 @@
 #### Breaking Changes
 
 #### Bugs Fixed
+* Fixed regression where `user_agent_overwrite` kwarg was not cleaned up properly, causing `TypeError` crash on sync client construction. See [PR 45653](https://github.com/Azure/azure-sdk-for-python/pull/45653)
 
 #### Other Changes
 
diff --git a/sdk/cosmos/azure-cosmos/azure/cosmos/_cosmos_client_connection.py b/sdk/cosmos/azure-cosmos/azure/cosmos/_cosmos_client_connection.py
index 3ffe52448d14..d1f628c1db31 100644
--- a/sdk/cosmos/azure-cosmos/azure/cosmos/_cosmos_client_connection.py
+++ b/sdk/cosmos/azure-cosmos/azure/cosmos/_cosmos_client_connection.py
@@ -1,4 +1,4 @@
-# The MIT License (MIT)
+# The MIT License (MIT)
 # Copyright (c) 2014 Microsoft Corporation
 
 # Permission is hereby granted, free of charge, to any person obtaining a copy
@@ -248,6 +248,7 @@ def __init__( # pylint: disable=too-many-statements
         ]
         # after passing in the user_agent into the user agent policy the user_agent is no longer needed
         kwargs.pop("user_agent", None)
+        kwargs.pop("user_agent_overwrite", None)
 
         transport = kwargs.pop("transport", None)
         self.pipeline_client: PipelineClient[HttpRequest, HttpResponse] = PipelineClient(
diff --git a/sdk/cosmos/azure-cosmos/azure/cosmos/aio/_cosmos_client_connection_async.py b/sdk/cosmos/azure-cosmos/azure/cosmos/aio/_cosmos_client_connection_async.py
index f12a8ab50e70..df06a4a554a0 100644
--- a/sdk/cosmos/azure-cosmos/azure/cosmos/aio/_cosmos_client_connection_async.py
+++ b/sdk/cosmos/azure-cosmos/azure/cosmos/aio/_cosmos_client_connection_async.py
@@ -245,6 +245,7 @@ def __init__( # pylint: disable=too-many-statements
         ]
         # after passing in the user_agent into the user agent policy the user_agent is no longer needed
         kwargs.pop("user_agent", None)
+        kwargs.pop("user_agent_overwrite", None)
 
         transport = kwargs.pop("transport", None)
         self.pipeline_client: AsyncPipelineClient[HttpRequest, AsyncHttpResponse] = AsyncPipelineClient(
diff --git a/sdk/cosmos/azure-cosmos/tests/test_user_agent.py b/sdk/cosmos/azure-cosmos/tests/test_user_agent.py
index 41beb3cde109..c57cf09880ce 100644
--- a/sdk/cosmos/azure-cosmos/tests/test_user_agent.py
+++ b/sdk/cosmos/azure-cosmos/tests/test_user_agent.py
@@ -95,6 +95,22 @@ def _run_case(use_suffix: bool) -> None:
         _run_case(False)  # prefix scenario
         _run_case(True)   # suffix scenario
 
+    def test_user_agent_overwrite_does_not_leak_sync(self):
+        """Regression test: user_agent_overwrite must not leak into the HTTP transport kwargs.
+        """
+        # user_agent + user_agent_overwrite=True at client level
+        self._check({'user_agent': 'MyApp/1.0', 'user_agent_overwrite': True})
+        # user_agent + user_agent_overwrite=False at client level
+        self._check({'user_agent': 'MyApp/1.0', 'user_agent_overwrite': False})
+        # user_agent_suffix + user_agent_overwrite=True at client level
+        self._check({'user_agent_suffix': 'MyApp/1.0', 'user_agent_overwrite': True})
+        # user_agent_suffix + user_agent_overwrite=False at client level
+        self._check({'user_agent_suffix': 'MyApp/1.0', 'user_agent_overwrite': False})
+        # user_agent_overwrite=True alone (no custom user_agent string)
+        self._check({'user_agent_overwrite': True})
+        # user_agent_overwrite=False alone
+        self._check({'user_agent_overwrite': False})
+
 
 if __name__ == '__main__':
     unittest.main()
diff --git a/sdk/cosmos/azure-cosmos/tests/test_user_agent_async.py b/sdk/cosmos/azure-cosmos/tests/test_user_agent_async.py
index 507a3f1e81b0..af6cd1ba9486 100644
--- a/sdk/cosmos/azure-cosmos/tests/test_user_agent_async.py
+++ b/sdk/cosmos/azure-cosmos/tests/test_user_agent_async.py
@@ -84,7 +84,22 @@ async def _run_case(use_suffix: bool):
         await _run_case(False)
         await _run_case(True)
 
+    async def test_user_agent_overwrite_does_not_leak_async(self):
+        """Regression test: user_agent_overwrite must not leak into the HTTP transport kwargs.
+        """
+        # user_agent + user_agent_overwrite=True at client level
+        await self._check({'user_agent': 'MyApp/1.0', 'user_agent_overwrite': True})
+        # user_agent + user_agent_overwrite=False at client level
+        await self._check({'user_agent': 'MyApp/1.0', 'user_agent_overwrite': False})
+        # user_agent_suffix + user_agent_overwrite=True at client level
+        await self._check({'user_agent_suffix': 'MyApp/1.0', 'user_agent_overwrite': True})
+        # user_agent_suffix + user_agent_overwrite=False at client level
+        await self._check({'user_agent_suffix': 'MyApp/1.0', 'user_agent_overwrite': False})
+        # user_agent_overwrite=True alone (no custom user_agent string)
+        await self._check({'user_agent_overwrite': True})
+        # user_agent_overwrite=False alone
+        await self._check({'user_agent_overwrite': False})
+
 
 if __name__ == "__main__":
     unittest.main()
-