diff --git a/CHANGELOG.md b/CHANGELOG.md
index 2bc3b85..9572383 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,3 +1,7 @@
+## 10.1.0
+
+* BaseAPIClient: add request_session constructor argument
+
 ## 10.0.1
 
 * Fix get_received_texts_iterator() method
diff --git a/notifications_python_client/__init__.py b/notifications_python_client/__init__.py
index 436df44..887bf8d 100644
--- a/notifications_python_client/__init__.py
+++ b/notifications_python_client/__init__.py
@@ -7,7 +7,7 @@
 #
 # -- http://semver.org/
 
-__version__ = "10.0.1"
+__version__ = "10.1.0"
 
 from notifications_python_client.errors import (  # noqa
     REQUEST_ERROR_MESSAGE,
diff --git a/notifications_python_client/base.py b/notifications_python_client/base.py
index 333c346..49a4326 100644
--- a/notifications_python_client/base.py
+++ b/notifications_python_client/base.py
@@ -13,7 +13,13 @@
 
 
 class BaseAPIClient:
-    def __init__(self, api_key, base_url="https://api.notifications.service.gov.uk", timeout=30):
+    def __init__(
+        self,
+        api_key,
+        base_url="https://api.notifications.service.gov.uk",
+        timeout=30,
+        request_session=None,
+    ):
         """
         Initialise the client
         Error if either of base_url or secret missing
@@ -32,7 +38,7 @@ def __init__(self, api_key, base_url="https://api.notifications.service.gov.uk",
         self.service_id = service_id
         self.api_key = api_key
         self.timeout = timeout
-        self.request_session = requests.Session()
+        self.request_session = request_session or requests.Session()
 
     def put(self, url, data):
         return self.request("PUT", url, data=data)