diff --git a/massive/rest/__init__.py b/massive/rest/__init__.py
index 5a00da5a..d1f6aa2c 100644
--- a/massive/rest/__init__.py
+++ b/massive/rest/__init__.py
@@ -20,7 +20,7 @@
     ContractsClient,
 )
 from .vX import VXClient
-from typing import Optional, Any
+from typing import Optional, Any, Dict
 import os
 
 BASE = "https://api.massive.com"
@@ -60,6 +60,7 @@ def __init__(
         verbose: bool = False,
         trace: bool = False,
         custom_json: Optional[Any] = None,
+        connection_pool_kw: Optional[Dict[str, Any]] = None,
     ):
         super().__init__(
             api_key=api_key,
@@ -72,6 +73,7 @@ def __init__(
             verbose=verbose,
             trace=trace,
             custom_json=custom_json,
+            connection_pool_kw=connection_pool_kw,
         )
         self.vx = VXClient(
             api_key=api_key,
@@ -84,4 +86,5 @@ def __init__(
             verbose=verbose,
             trace=trace,
             custom_json=custom_json,
+            connection_pool_kw=connection_pool_kw,
         )
diff --git a/massive/rest/base.py b/massive/rest/base.py
index 3349d7ef..dadead3f 100644
--- a/massive/rest/base.py
+++ b/massive/rest/base.py
@@ -28,6 +28,7 @@ def __init__(
         connect_timeout: float,
         read_timeout: float,
         num_pools: int,
+        connection_pool_kw: Optional[Dict[str, Any]],
         retries: int,
         base: str,
         pagination: bool,
@@ -80,6 +81,7 @@ def __init__(
             cert_reqs="CERT_REQUIRED",
             retries=retry_strategy,  # use the customized Retry instance
             timeout=self.timeout,  # set timeout for each request
+            **(connection_pool_kw if connection_pool_kw is not None else {}),
         )
 
         if verbose:
diff --git a/test_rest/test_connection_pool_kw.py b/test_rest/test_connection_pool_kw.py
new file mode 100644
index 00000000..d1ed0b31
--- /dev/null
+++ b/test_rest/test_connection_pool_kw.py
@@ -0,0 +1,36 @@
+import sys
+import os
+
+sys.path.insert(0, os.path.dirname(__file__))
+
+import unittest
+from unittest.mock import patch, MagicMock
+from massive import RESTClient
+
+
+class ConnectionPoolKwTest(unittest.TestCase):
+    def _make_client(self, **kwargs):
+        with patch("urllib3.PoolManager") as mock_pm:
+            mock_pm.return_value = MagicMock()
+            RESTClient(api_key="test", **kwargs)
+            return mock_pm
+
+    def test_default_no_extra_kwargs(self):
+        """connection_pool_kw=None passes no extra kwargs to PoolManager."""
+        mock_pm = self._make_client()
+        _, call_kwargs = mock_pm.call_args
+        self.assertNotIn("connection_pool_kw", call_kwargs)
+
+    def test_connection_pool_kw_passed_as_kwargs(self):
+        """connection_pool_kw dict is unpacked into PoolManager as kwargs."""
+        extra = {"maxsize": 20, "block": True}
+        mock_pm = self._make_client(connection_pool_kw=extra)
+        _, call_kwargs = mock_pm.call_args
+        self.assertEqual(call_kwargs["maxsize"], 20)
+        self.assertEqual(call_kwargs["block"], True)
+
+    def test_empty_connection_pool_kw(self):
+        """Empty dict connection_pool_kw adds no extra kwargs to PoolManager."""
+        mock_pm = self._make_client(connection_pool_kw={})
+        _, call_kwargs = mock_pm.call_args
+        self.assertNotIn("connection_pool_kw", call_kwargs)