diff --git a/sdks/python/apache_beam/runners/portability/prism_runner_test.py b/sdks/python/apache_beam/runners/portability/prism_runner_test.py
index a65f9a9960b4..9c1620603fd3 100644
--- a/sdks/python/apache_beam/runners/portability/prism_runner_test.py
+++ b/sdks/python/apache_beam/runners/portability/prism_runner_test.py
@@ -19,7 +19,10 @@
 import argparse
 import logging
 import os.path
+import queue
 import shlex
+import threading
+import time
 import typing
 import unittest
 import zipfile
@@ -37,8 +40,10 @@
 from apache_beam.options.pipeline_options import PortableOptions
 from apache_beam.options.pipeline_options import StandardOptions
 from apache_beam.options.pipeline_options import TypeOptions
+from apache_beam.portability.api import beam_fn_api_pb2
 from apache_beam.runners.portability import portable_runner_test
 from apache_beam.runners.portability import prism_runner
+from apache_beam.runners.worker import worker_pool_main
 from apache_beam.testing.util import assert_that
 from apache_beam.testing.util import equal_to
 from apache_beam.transforms import trigger
@@ -488,6 +493,63 @@ def test_singleton(self, enable_singleton):
       else:
         mock_prism_server.assert_called_once()
 
+  def test_loopback_worker_daemon_thread_accumulation(self):
+    """Verifies that in LOOPBACK mode, the external worker pool servicer properly
+    tracks active thread-based SdkHarness workers and cleanly shuts them down in
+    StopWorker via sentinel messages. This prevents background daemon threads from
+    accumulating across sequential pipeline executions and leaking resources.
+    """
+    servicer = worker_pool_main.BeamFnExternalWorkerPoolServicer(
+        use_process=False, state_cache_size=0, data_buffer_time_limit_ms=0)
+
+    active_workers = []
+    mock_responses = queue.Queue()
+
+    def mock_run(self_worker):
+      active_workers.append(self_worker)
+      mock_responses.get()
+      active_workers.remove(self_worker)
+
+    def wait_for_workers(expected_count, timeout=5.0):
+      start = time.time()
+      while time.time() - start < timeout:
+        if len(active_workers) == expected_count:
+          return
+        time.sleep(0.01)
+      self.assertEqual(len(active_workers), expected_count)
+
+    with mock.patch(
+        'apache_beam.runners.worker.sdk_worker.SdkHarness') as mock_harness:
+      mock_harness.return_value._responses = mock_responses
+      mock_harness.return_value.run = lambda: mock_run(mock_harness)
+
+      # Simulate starting Worker 1 for Pipeline 1
+      req1 = beam_fn_api_pb2.StartWorkerRequest(worker_id="worker_1")
+      req1.control_endpoint.url = "localhost:12345"
+      servicer.StartWorker(req1, None)
+
+      wait_for_workers(1)
+
+      # Simulate stopping Worker 1 at the end of Pipeline 1
+      stop_req1 = beam_fn_api_pb2.StopWorkerRequest(worker_id="worker_1")
+      servicer.StopWorker(stop_req1, None)
+
+      # Verify the fix: StopWorker successfully tells the thread harness to shut down,
+      # completely resolving the thread leak!
+      wait_for_workers(0)
+
+      # Simulate starting Worker 2 for Pipeline 2
+      req2 = beam_fn_api_pb2.StartWorkerRequest(worker_id="worker_2")
+      req2.control_endpoint.url = "localhost:12345"
+      servicer.StartWorker(req2, None)
+
+      wait_for_workers(1)
+
+      # Clean up the second worker
+      servicer.StopWorker(
+          beam_fn_api_pb2.StopWorkerRequest(worker_id="worker_2"), None)
+      wait_for_workers(0)
+
 
 if __name__ == '__main__':
   # Run the tests.
diff --git a/sdks/python/apache_beam/runners/worker/worker_pool_main.py b/sdks/python/apache_beam/runners/worker/worker_pool_main.py
index 425a9fc57752..efe927b729c1 100644
--- a/sdks/python/apache_beam/runners/worker/worker_pool_main.py
+++ b/sdks/python/apache_beam/runners/worker/worker_pool_main.py
@@ -45,6 +45,7 @@
 from apache_beam.portability.api import beam_fn_api_pb2_grpc
 from apache_beam.runners.worker import sdk_worker
 from apache_beam.utils import thread_pool_executor
+from apache_beam.utils.sentinel import Sentinel
 
 _LOGGER = logging.getLogger(__name__)
 
@@ -82,6 +83,7 @@ def __init__(
     self._state_cache_size = state_cache_size
     self._data_buffer_time_limit_ms = data_buffer_time_limit_ms
     self._worker_processes: dict[str, subprocess.Popen] = {}
+    self._worker_threads: dict[str, sdk_worker.SdkHarness] = {}
 
   @classmethod
   def start(
@@ -166,6 +168,7 @@ def StartWorker(
             worker_id=start_worker_request.worker_id,
             state_cache_size=self._state_cache_size,
             data_buffer_time_limit_ms=self._data_buffer_time_limit_ms)
+        self._worker_threads[start_worker_request.worker_id] = worker
         worker_thread = threading.Thread(
             name='run_worker_%s' % start_worker_request.worker_id,
             target=worker.run)
@@ -188,6 +191,14 @@ def StopWorker(
       _LOGGER.info("Stopping worker %s" % stop_worker_request.worker_id)
       kill_process_gracefully(worker_process)
 
+    # applicable for thread mode to ensure thread cleanup by
+    # unblocking the harness request stream.
+    worker_thread_harness = self._worker_threads.pop(
+        stop_worker_request.worker_id, None)
+    if worker_thread_harness:
+      _LOGGER.info("Stopping thread worker %s" % stop_worker_request.worker_id)
+      worker_thread_harness._responses.put(Sentinel.sentinel)
+
     return beam_fn_api_pb2.StopWorkerResponse()