diff --git a/django/db/models/fields/related_descriptors.py b/django/db/models/fields/related_descriptors.py
index a9445d5d10..451d0b4e1f 100644
--- a/django/db/models/fields/related_descriptors.py
+++ b/django/db/models/fields/related_descriptors.py
@@ -999,7 +999,8 @@ def create_forward_many_to_many_manager(superclass, rel, reverse):
                     for obj in objs:
                         fk_val = (
                             self.target_field.get_foreign_related_value(obj)[0]
-                            if isinstance(obj, self.model) else obj
+                            if isinstance(obj, self.model)
+                            else self.target_field.get_prep_value(obj)
                         )
                         if fk_val in old_ids:
                             old_ids.remove(fk_val)
diff --git a/tests/many_to_many/tests.py b/tests/many_to_many/tests.py
index 79d0fa420c..b4314580ec 100644
--- a/tests/many_to_many/tests.py
+++ b/tests/many_to_many/tests.py
@@ -469,6 +469,19 @@ class ManyToManyTests(TestCase):
         self.a4.publications.set([], clear=True)
         self.assertQuerysetEqual(self.a4.publications.all(), [])
 
+    def test_set_existing_different_type(self):
+        # Existing many-to-many relations remain the same for values provided
+        # with a different type.
+        ids = set(Publication.article_set.through.objects.filter(
+            article__in=[self.a4, self.a3],
+            publication=self.p2,
+        ).values_list('id', flat=True))
+        self.p2.article_set.set([str(self.a4.pk), str(self.a3.pk)])
+        new_ids = set(Publication.article_set.through.objects.filter(
+            publication=self.p2,
+        ).values_list('id', flat=True))
+        self.assertEqual(ids, new_ids)
+
     def test_assign_forward(self):
         msg = (
             "Direct assignment to the reverse side of a many-to-many set is "