diff --git a/moto/s3/models.py b/moto/s3/models.py
index 33b5c503b..abc80a3ab 100644
--- a/moto/s3/models.py
+++ b/moto/s3/models.py
@@ -1421,6 +1421,18 @@ class S3Backend(BaseBackend, CloudWatchMetricProvider):
             "s3::keyrestore", transition={"progression": "immediate"}
         )
 
+    def reset(self):
+        # For every key and multipart, Moto opens a TemporaryFile to write the value of those keys
+        # Ensure that these TemporaryFile-objects are closed, and leave no filehandles open
+        for bucket in self.buckets.values():
+            for key in bucket.keys.values():
+                if isinstance(key, FakeKey):
+                    key._value_buffer.close()
+                    if key.multipart is not None:
+                        for part in key.multipart.parts.values():
+                            part._value_buffer.close()
+        super().reset()
+
     @property
     def _url_module(self):
         # The urls-property can be different depending on env variables
diff --git a/tests/test_s3/test_s3_multipart.py b/tests/test_s3/test_s3_multipart.py
index b152c88ba..2226464ca 100644
--- a/tests/test_s3/test_s3_multipart.py
+++ b/tests/test_s3/test_s3_multipart.py
@@ -1,8 +1,10 @@
 import boto3
 import os
+import gc
 import pytest
 import sure  # noqa # pylint: disable=unused-import
 import requests
+import warnings
 
 from botocore.client import ClientError
 from functools import wraps
@@ -1009,3 +1011,39 @@ def test_head_object_returns_part_count():
     # Header is not returned when we do not pass PartNumber
     resp = client.head_object(Bucket=bucket, Key=key)
     resp.shouldnt.have.key("PartsCount")
+
+
+def test_reset_after_multipart_upload_closes_file_handles():
+    """
+    Large Uploads are written to disk for performance reasons
+    This test verifies that the filehandles are properly closed when resetting Moto
+    """
+    s3 = s3model.S3Backend("us-west-1", "1234")
+    s3.create_bucket("my-bucket", "us-west-1")
+    s3.put_object("my-bucket", "my-key", "x" * 10_000_000)
+
+    with warnings.catch_warnings(record=True) as warning_list:
+        warnings.simplefilter("always", ResourceWarning)  # add filter
+        s3.reset()
+        gc.collect()
+    warning_types = [type(warn.message) for warn in warning_list]
+    warning_types.shouldnt.contain(ResourceWarning)
+
+
+def test_deleting_large_upload_closes_file_handles():
+    """
+    Large Uploads are written to disk for performance reasons
+    This test verifies that the filehandles are properly closed on deletion
+    """
+    s3 = s3model.S3Backend("us-east-1", "1234")
+    s3.create_bucket("my-bucket", "us-west-1")
+    s3.put_object("my-bucket", "my-key", "x" * 10_000_000)
+
+    with warnings.catch_warnings(record=True) as warning_list:
+        warnings.simplefilter("always", ResourceWarning)  # add filter
+        s3.delete_object(bucket_name="my-bucket", key_name="my-key")
+        gc.collect()
+    warning_types = [type(warn.message) for warn in warning_list]
+    warning_types.shouldnt.contain(ResourceWarning)
+
+    s3.reset()