diff --git a/sdks/python/apache_beam/yaml/standard_io.yaml b/sdks/python/apache_beam/yaml/standard_io.yaml
index e62b3a562c30..d510d73562c5 100644
--- a/sdks/python/apache_beam/yaml/standard_io.yaml
+++ b/sdks/python/apache_beam/yaml/standard_io.yaml
@@ -145,6 +145,7 @@
         path: 'path'
       'WriteToJson':
         path: 'path'
+        num_shards: 'num_shards'
       'ReadFromParquet':
         path: 'file_pattern'
       'WriteToParquet':
diff --git a/sdks/python/apache_beam/yaml/yaml_mapping.py b/sdks/python/apache_beam/yaml/yaml_mapping.py
index a6b2b5704751..69b38db4173c 100644
--- a/sdks/python/apache_beam/yaml/yaml_mapping.py
+++ b/sdks/python/apache_beam/yaml/yaml_mapping.py
@@ -16,8 +16,15 @@
 #
 
 """This module defines the basic MapToFields operation."""
+import atexit
+import importlib
 import itertools
+import os
+import queue
 import re
+import sys
+import threading
+import uuid
 from collections import abc
 from collections.abc import Callable
 from collections.abc import Collection
@@ -53,14 +60,6 @@
 from apache_beam.yaml.yaml_errors import maybe_with_exception_handling_transform_fn
 from apache_beam.yaml.yaml_provider import dicts_to_rows
 
-# Import js2py package if it exists
-try:
-  import js2py
-  from js2py.base import JsObjectWrapper
-except ImportError:
-  js2py = None
-  JsObjectWrapper = object
-
 _str_expression_fields = {
     'AssignTimestamps': 'timestamp',
     'Filter': 'keep',
@@ -178,20 +177,6 @@ def _check_mapping_arguments(
     raise ValueError(f'{transform_name} cannot specify "name" without "path"')
 
 
-# js2py's JsObjectWrapper object has a self-referencing __dict__ property
-# that cannot be pickled without implementing the __getstate__ and
-# __setstate__ methods.
-class _CustomJsObjectWrapper(JsObjectWrapper):
-  def __init__(self, js_obj):
-    super().__init__(js_obj.__dict__['_obj'])
-
-  def __getstate__(self):
-    return self.__dict__.copy()
-
-  def __setstate__(self, state):
-    self.__dict__.update(state)
-
-
 # TODO(yaml) Improve type inferencing for JS UDF's
 def py_value_to_js_dict(py_value):
   if ((isinstance(py_value, tuple) and hasattr(py_value, '_asdict')) or
@@ -205,85 +190,181 @@ def py_value_to_js_dict(py_value):
     return py_value
 
 
-# TODO(yaml) Consider adding optional language version parameter to support
-#  ECMAScript 5 and 6
+class PythonMonkeyDispatcher:
+  """Dispatcher for executing JavaScript code using pythonmonkey.
+
+  This class manages a worker thread to execute JavaScript, ensuring that
+  pythonmonkey is only imported and used within that thread. It also handles
+  process shutdown carefully to avoid segmentation faults known to occur
+  when pythonmonkey is present during standard Python interpreter finalization.
+  """
+  def __init__(self):
+    self._req_queue = queue.Queue()
+    self._resp_events = {}
+    self._resp_data = {}
+    self._lock = threading.Lock()
+    self._thread = threading.Thread(target=self._worker, daemon=True)
+    self._started = False
+    # Register the stop method to be called on exit.
+    # atexit handlers are executed in LIFO order. By registering at import time,
+    # we ensure this handler runs last, allowing other cleanup handlers
+    # (registered later) to execute first.
+    atexit.register(self.stop)
+
+  def start(self):
+    with self._lock:
+      if not self._started:
+        self._thread.start()
+        self._started = True
+
+  def stop(self):
+    # This method is called on process exit.
+    if not self._started:
+      return
+    # Flush standard streams before forced exit to avoid data loss.
+    try:
+      sys.stdout.flush()
+      sys.stderr.flush()
+    except Exception:
+      pass
+    # Force an immediate exit to avoid a segmentation fault that occurs with
+    # pythonmonkey during standard interpreter finalization.
+    # Since this runs as one of the last atexit handlers (due to import-time
+    # registration), most other cleanup should have already completed.
+    os._exit(0)
+
+  def _worker(self):
+    try:
+      import pythonmonkey as pm
+    except ImportError:
+      pm = None
+
+    self._pm = pm
+    self._cache = {}
+
+    while True:
+      req = self._req_queue.get()
+      if req is None:
+        break
+
+      req_id, type_str, payload = req
+      res = None
+      is_err = False
+      try:
+        if self._pm is None:
+          raise ImportError(
+              "PythonMonkey not installed or failed to import in worker thread."
+          )
+
+        if type_str == 'exec':
+          source, row = payload
+          if source not in self._cache:
+            self._cache[source] = self._pm.eval(f"({source})")
+          func = self._cache[source]
+          res = func(row)
+      except Exception as e:
+        res = e
+        is_err = True
+
+      with self._lock:
+        if req_id in self._resp_events:
+          self._resp_data[req_id] = (is_err, res)
+          self._resp_events[req_id].set()
+
+  def eval_and_run(self, source, row):
+    if not self._started:
+      self.start()
+
+    req_id = str(uuid.uuid4())
+    event = threading.Event()
+    with self._lock:
+      self._resp_events[req_id] = event
+
+    self._req_queue.put((req_id, 'exec', (source, row)))
+    event.wait()
+
+    with self._lock:
+      is_err, result = self._resp_data.pop(req_id)
+      del self._resp_events[req_id]
+
+    if is_err:
+      raise result
+    return result
+
+
+_pythonmonkey_dispatcher = PythonMonkeyDispatcher()
+
+
+class JavaScriptCallable:
+  def __init__(self, source, name=None):
+    self._source = source
+    self._name = name
+
+  def __call__(self, row):
+    # Check for pythonmonkey availability lazily (on first call)
+    if importlib.util.find_spec("pythonmonkey") is None:
+      raise RuntimeError(
+          "PythonMonkey is not installed. Please install 'apache_beam[yaml]' "
+          "to use JavaScript mapping functions.")
+
+    row_as_dict = py_value_to_js_dict(row)
+    try:
+      # If we have a name, it means we evaluated a file and need to call
+      # a specific function.
+      # Dispatcher expects a self-contained source/expression.
+      if self._name:
+        # Wrap: (function() { <source>; return <name>; })()
+        effective_source = (
+            f"(function() {{ {self._source}; return {self._name}; }})()")
+      else:
+        # Expression/Callable case: Wrap in parens to be safe
+        effective_source = f"({self._source})"
+
+      js_result = _pythonmonkey_dispatcher.eval_and_run(
+          effective_source, row_as_dict)
+
+    except Exception as exn:
+      raise RuntimeError(
+          f"Error evaluating javascript expression: {exn}") from exn
+    return dicts_to_rows(_finalize_js_result(js_result))
+
+
+def _finalize_js_result(obj):
+  """Coerce pythonmonkey objects to native Python objects (specifically
+  strings).
+  """
+  if isinstance(obj, str):
+    return str(obj)
+  if isinstance(obj, list):
+    return [_finalize_js_result(x) for x in obj]
+  if isinstance(obj, dict):
+    return {k: _finalize_js_result(v) for k, v in obj.items()}
+  return obj
+
+
 def _expand_javascript_mapping_func(
     original_fields, expression=None, callable=None, path=None, name=None):
 
-  # Check for installed js2py package
-  if js2py is None:
+  if importlib.util.find_spec("pythonmonkey") is None:
     raise ValueError(
-        "Javascript mapping functions are not supported on"
-        " Python 3.12 or later.")
-
-  # import remaining js2py objects
-  from js2py import base
-  from js2py.constructors import jsdate
-  from js2py.internals import simplex
-
-  js_array_type = (
-      base.PyJsArray,
-      base.PyJsArrayBuffer,
-      base.PyJsInt8Array,
-      base.PyJsUint8Array,
-      base.PyJsUint8ClampedArray,
-      base.PyJsInt16Array,
-      base.PyJsUint16Array,
-      base.PyJsInt32Array,
-      base.PyJsUint32Array,
-      base.PyJsFloat32Array,
-      base.PyJsFloat64Array)
-
-  def _js_object_to_py_object(obj):
-    if isinstance(obj, (base.PyJsNumber, base.PyJsString, base.PyJsBoolean)):
-      return base.to_python(obj)
-    elif isinstance(obj, js_array_type):
-      return [_js_object_to_py_object(value) for value in obj.to_list()]
-    elif isinstance(obj, jsdate.PyJsDate):
-      return obj.to_utc_dt()
-    elif isinstance(obj, (base.PyJsNull, base.PyJsUndefined)):
-      return None
-    elif isinstance(obj, base.PyJsError):
-      raise RuntimeError(obj['message'])
-    elif isinstance(obj, base.PyJsObject):
-      return {
-          key: _js_object_to_py_object(value['value'])
-          for (key, value) in obj.own.items()
-      }
-    elif isinstance(obj, base.JsObjectWrapper):
-      return _js_object_to_py_object(obj._obj)
-
-    return obj
+        "PythonMonkey is not installed. Please install 'apache_beam[yaml]' "
+        "to use JavaScript mapping functions.")
 
   if expression:
     source = '\n'.join(['function(__row__) {'] + [
         f'  {name} = __row__.{name}'
         for name in original_fields if name in expression
     ] + ['  return (' + expression + ')'] + ['}'])
-    js_func = _CustomJsObjectWrapper(js2py.eval_js(source))
+    return JavaScriptCallable(source)
 
   elif callable:
-    js_func = _CustomJsObjectWrapper(js2py.eval_js(callable))
+    return JavaScriptCallable(callable)
 
   else:
     if not path.endswith('.js'):
       raise ValueError(f'File "{path}" is not a valid .js file.')
     udf_code = FileSystems.open(path).read().decode()
-    js = js2py.EvalJs()
-    js.eval(udf_code)
-    js_func = _CustomJsObjectWrapper(getattr(js, name))
-
-  def js_wrapper(row):
-    row_as_dict = py_value_to_js_dict(row)
-    try:
-      js_result = js_func(row_as_dict)
-    except simplex.JsException as exn:
-      raise RuntimeError(
-          f"Error evaluating javascript expression: "
-          f"{exn.mes['message']}") from exn
-    return dicts_to_rows(_js_object_to_py_object(js_result))
-
-  return js_wrapper
+    return JavaScriptCallable(udf_code, name=name)
 
 
 def _expand_python_mapping_func(
diff --git a/sdks/python/apache_beam/yaml/yaml_udf_test.py b/sdks/python/apache_beam/yaml/yaml_udf_test.py
index 3d664ab9de41..e6d0a0af41a2 100644
--- a/sdks/python/apache_beam/yaml/yaml_udf_test.py
+++ b/sdks/python/apache_beam/yaml/yaml_udf_test.py
@@ -14,6 +14,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 #
+import importlib
 import logging
 import os
 import shutil
@@ -31,11 +32,17 @@
 from apache_beam.yaml.yaml_provider import dicts_to_rows
 from apache_beam.yaml.yaml_transform import YamlTransform
 
+# We use find_spec to check for pythonmonkey availability without importing it.
+# Importing pythonmonkey initializes the engine and binds it to the current
+# thread (MainThread). This causes "too much recursion" errors when the
+# Dispatcher later tries to use it from a background thread.
 try:
-  import js2py
+  pm_available = importlib.util.find_spec("pythonmonkey") is not None
 except ImportError:
-  js2py = None
-  logging.warning('js2py is not installed; some tests will be skipped.')
+  pm_available = False
+
+if not pm_available:
+  logging.warning('pythonmonkey is not installed; some tests will be skipped.')
 
 
 def as_rows():
@@ -63,7 +70,7 @@ def setUp(self):
   def tearDown(self):
     shutil.rmtree(self.tmpdir)
 
-  @unittest.skipIf(js2py is None, 'js2py not installed.')
+  @unittest.skipIf(not pm_available, 'pythonmonkey not installed.')
   def test_map_to_fields_filter_inline_js(self):
     with beam.Pipeline(options=beam.options.pipeline_options.PipelineOptions(
         pickle_library='cloudpickle', yaml_experimental_features=['javascript'
@@ -197,7 +204,7 @@ def test_map_to_fields_sql_reserved_keyword_append():
               beam.Row(label='389a', timestamp=2, label_copy="389a"),
           ]))
 
-  @unittest.skipIf(js2py is None, 'js2py not installed.')
+  @unittest.skipIf(not pm_available, 'pythonmonkey not installed.')
   def test_filter_inline_js(self):
     with beam.Pipeline(options=beam.options.pipeline_options.PipelineOptions(
         pickle_library='cloudpickle', yaml_experimental_features=['javascript'
@@ -252,7 +259,7 @@ def test_filter_inline_py(self):
                   row=beam.Row(rank=2, values=[7, 8, 9])),
           ]))
 
-  @unittest.skipIf(js2py is None, 'js2py not installed.')
+  @unittest.skipIf(not pm_available, 'pythonmonkey not installed.')
   def test_filter_expression_js(self):
     with beam.Pipeline(options=beam.options.pipeline_options.PipelineOptions(
         pickle_library='cloudpickle', yaml_experimental_features=['javascript'
@@ -296,7 +303,7 @@ def test_filter_expression_py(self):
                   row=beam.Row(rank=0, values=[1, 2, 3])),
           ]))
 
-  @unittest.skipIf(js2py is None, 'js2py not installed.')
+  @unittest.skipIf(not pm_available, 'pythonmonkey not installed.')
   def test_filter_inline_js_file(self):
     data = '''
     function f(x) {
diff --git a/sdks/python/setup.py b/sdks/python/setup.py
index 3b195cdbc087..8500830a17b1 100644
--- a/sdks/python/setup.py
+++ b/sdks/python/setup.py
@@ -574,8 +574,9 @@ def get_portability_package_data():
               'docstring-parser>=0.15,<1.0',
               'jinja2>=3.0,<3.2',
               'virtualenv-clone>=0.5,<1.0',
-              # https://github.com/PiotrDabkowski/Js2Py/issues/317
-              'js2py>=0.74,<1; python_version<"3.12"',
+              # pythonmonkey is used for Javascript mapping support
+              # Please install NPM and Node.js before installing PythonMonkey.
+              'pythonmonkey>=1.3.0',
               'jsonschema>=4.0.0,<5.0.0',
           ] + dataframe_dependency,
           # Keep the following dependencies in line with what we test against