diff --git a/doc/changelog.rst b/doc/changelog.rst
index 6fffcdf696..c008066c22 100644
--- a/doc/changelog.rst
+++ b/doc/changelog.rst
@@ -42,6 +42,16 @@ PyMongo 4.9 brings a number of improvements including:
 - Fixed a bug where PyMongo would raise ``InvalidBSON: date value out of range``
   when using :attr:`~bson.codec_options.DatetimeConversion.DATETIME_CLAMP` or
   :attr:`~bson.codec_options.DatetimeConversion.DATETIME_AUTO` with a non-UTC timezone.
+- Added a warning to unclosed MongoClient instances
+  telling users to explicitly close clients when finished with them to avoid leaking resources.
+  For example:
+
+  .. code-block::
+
+    sys:1: ResourceWarning: Unclosed MongoClient opened at:
+        File "/Users/<user>/my_file.py", line 8, in <module>``
+            client = MongoClient()
+    Call MongoClient.close() to safely shut down your client and free up resources.
 - The default value for ``connect`` in ``MongoClient`` is changed to ``False`` when running on
   unction-as-a-service (FaaS) like AWS Lambda, Google Cloud Functions, and Microsoft Azure Functions.
   On some FaaS systems, there is a ``fork()`` operation at function
diff --git a/pymongo/asynchronous/mongo_client.py b/pymongo/asynchronous/mongo_client.py
index b5e73e8de8..a84fbf2e59 100644
--- a/pymongo/asynchronous/mongo_client.py
+++ b/pymongo/asynchronous/mongo_client.py
@@ -34,6 +34,7 @@
 
 import contextlib
 import os
+import warnings
 import weakref
 from collections import defaultdict
 from typing import (
@@ -871,6 +872,7 @@ def __init__(
         )
 
         self._opened = False
+        self._closed = False
         self._init_background()
 
         if _IS_SYNC and connect:
@@ -1180,6 +1182,22 @@ def __getitem__(self, name: str) -> database.AsyncDatabase[_DocumentType]:
         """
         return database.AsyncDatabase(self, name)
 
+    def __del__(self) -> None:
+        """Check that this AsyncMongoClient has been closed and issue a warning if not."""
+        try:
+            if not self._closed:
+                warnings.warn(
+                    (
+                        f"Unclosed {type(self).__name__} opened at:\n{self._topology_settings._stack}"
+                        f"Call {type(self).__name__}.close() to safely shut down your client and free up resources."
+                    ),
+                    ResourceWarning,
+                    stacklevel=2,
+                    source=self,
+                )
+        except AttributeError:
+            pass
+
     def _close_cursor_soon(
         self,
         cursor_id: int,
@@ -1547,6 +1565,7 @@ async def close(self) -> None:
         if self._encrypter:
             # TODO: PYTHON-1921 Encrypted MongoClients cannot be re-opened.
             await self._encrypter.close()
+        self._closed = True
 
     if not _IS_SYNC:
         # Add support for contextlib.aclosing.
diff --git a/pymongo/asynchronous/settings.py b/pymongo/asynchronous/settings.py
index c41c638e6c..1103e1bd18 100644
--- a/pymongo/asynchronous/settings.py
+++ b/pymongo/asynchronous/settings.py
@@ -82,7 +82,7 @@ def __init__(
         self._topology_id = ObjectId()
         # Store the allocation traceback to catch unclosed clients in the
         # test suite.
-        self._stack = "".join(traceback.format_stack())
+        self._stack = "".join(traceback.format_stack()[:-2])
 
     @property
     def seeds(self) -> Collection[tuple[str, int]]:
diff --git a/pymongo/synchronous/mongo_client.py b/pymongo/synchronous/mongo_client.py
index 26af488acd..cec78463b3 100644
--- a/pymongo/synchronous/mongo_client.py
+++ b/pymongo/synchronous/mongo_client.py
@@ -34,6 +34,7 @@
 
 import contextlib
 import os
+import warnings
 import weakref
 from collections import defaultdict
 from typing import (
@@ -871,6 +872,7 @@ def __init__(
         )
 
         self._opened = False
+        self._closed = False
         self._init_background()
 
         if _IS_SYNC and connect:
@@ -1180,6 +1182,22 @@ def __getitem__(self, name: str) -> database.Database[_DocumentType]:
         """
         return database.Database(self, name)
 
+    def __del__(self) -> None:
+        """Check that this MongoClient has been closed and issue a warning if not."""
+        try:
+            if not self._closed:
+                warnings.warn(
+                    (
+                        f"Unclosed {type(self).__name__} opened at:\n{self._topology_settings._stack}"
+                        f"Call {type(self).__name__}.close() to safely shut down your client and free up resources."
+                    ),
+                    ResourceWarning,
+                    stacklevel=2,
+                    source=self,
+                )
+        except AttributeError:
+            pass
+
     def _close_cursor_soon(
         self,
         cursor_id: int,
@@ -1543,6 +1561,7 @@ def close(self) -> None:
         if self._encrypter:
             # TODO: PYTHON-1921 Encrypted MongoClients cannot be re-opened.
             self._encrypter.close()
+        self._closed = True
 
     if not _IS_SYNC:
         # Add support for contextlib.closing.
diff --git a/pymongo/synchronous/settings.py b/pymongo/synchronous/settings.py
index 8719e86083..040776713f 100644
--- a/pymongo/synchronous/settings.py
+++ b/pymongo/synchronous/settings.py
@@ -82,7 +82,7 @@ def __init__(
         self._topology_id = ObjectId()
         # Store the allocation traceback to catch unclosed clients in the
         # test suite.
-        self._stack = "".join(traceback.format_stack())
+        self._stack = "".join(traceback.format_stack()[:-2])
 
     @property
     def seeds(self) -> Collection[tuple[str, int]]:
diff --git a/pyproject.toml b/pyproject.toml
index 225be8e1d8..19db00f19a 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -95,6 +95,9 @@ filterwarnings = [
     "module:please use dns.resolver.Resolver.resolve:DeprecationWarning",
     # https://github.com/dateutil/dateutil/issues/1314
     "module:datetime.datetime.utc:DeprecationWarning:dateutil",
+    # TODO: Remove both of these in https://jira.mongodb.org/browse/PYTHON-4731
+    "ignore:Unclosed AsyncMongoClient*",
+    "ignore:Unclosed MongoClient*",
 ]
 markers = [
     "auth_aws: tests that rely on pymongo-auth-aws",