diff --git a/django/forms/models.py b/django/forms/models.py
index 35ecdc1c0d5e..a8e6432f2a50 100644
--- a/django/forms/models.py
+++ b/django/forms/models.py
@@ -756,7 +756,7 @@ def save_new(self, form, commit=True):
         """Save and return a new model instance for the given form."""
         return form.save(commit=commit)
 
-    def save_existing(self, form, instance, commit=True):
+    def save_existing(self, form, obj, commit=True):
         """Save and return an existing model instance for the given form."""
         return form.save(commit=commit)
 
diff --git a/docs/releases/5.0.txt b/docs/releases/5.0.txt
index 8146f3bb67cd..a6f827e472f7 100644
--- a/docs/releases/5.0.txt
+++ b/docs/releases/5.0.txt
@@ -240,7 +240,8 @@ backends.
 Miscellaneous
 -------------
 
-* ...
+* The ``instance`` argument of the undocumented
+  ``BaseModelFormSet.save_existing()`` method is renamed to ``obj``.
 
 .. _deprecated-features-5.0: