diff --git a/src/requests/models.py b/src/requests/models.py
index c4b25fa079..8e190db319 100644
--- a/src/requests/models.py
+++ b/src/requests/models.py
@@ -563,8 +563,8 @@ def prepare_body(self, data, files, json=None):
 
             self.prepare_content_length(body)
 
-            # Add content-type if it wasn't explicitly provided.
-            if content_type and ("content-type" not in self.headers):
+            # Add content-type if it wasn't explicitly provided, or it was explicitly unset
+            if content_type and self.headers.get("content-type") is None:
                 self.headers["Content-Type"] = content_type
 
         self.body = body
diff --git a/tests/test_requests.py b/tests/test_requests.py
index 75d2deff2e..e2e72b91ab 100644
--- a/tests/test_requests.py
+++ b/tests/test_requests.py
@@ -1852,6 +1852,18 @@ def test_can_send_objects_with_files(self, httpbin, files):
         p = r.prepare()
         assert "multipart/form-data" in p.headers["Content-Type"]
 
+    def test_can_override_session_content_type_with_multipart_request(self, httpbin):
+        s = requests.Session()
+        s.headers["Content-Type"] = "application/json"
+        data = {"a": "this is a string"}
+        files = {"b": "foo"}
+        headers = {"Content-Type": None}
+        r = requests.Request(
+            "POST", httpbin("post"), headers=headers, data=data, files=files
+        )
+        p = s.prepare_request(r)
+        assert "multipart/form-data" in p.headers["Content-Type"]
+
     def test_can_send_file_object_with_non_string_filename(self, httpbin):
         f = io.BytesIO()
         f.name = 2