diff --git a/django/contrib/admin/options.py b/django/contrib/admin/options.py
index 6c202c8e613c..9a32d4e0d4f6 100644
--- a/django/contrib/admin/options.py
+++ b/django/contrib/admin/options.py
@@ -414,11 +414,35 @@ def get_ordering(self, request):
         """
         return self.ordering or ()  # otherwise we might try to *None, which is bad ;)
 
+    def _get_readonly_manual_pk_fields(self, obj):
+        """
+        Returns a list of primary key field names that should be readonly if:
+        - we're editing an existing object,
+        - the PK was manually defined (not auto-created),
+        - and it's not already in self.readonly_fields.
+        Supports both single and composite PKs.
+        """
+        if not obj or hasattr(self, "parent_model"):
+            return []
+
+        readonly = []
+        pk_fields = self.model._meta.pk_fields
+
+        for pk in pk_fields:
+            if (
+                pk.editable
+                and not pk.auto_created
+                and pk.name not in self.readonly_fields
+            ):
+                readonly.append(pk.name)
+
+        return readonly
+
     def get_readonly_fields(self, request, obj=None):
         """
         Hook for specifying custom readonly fields.
         """
-        return self.readonly_fields
+        return list(self.readonly_fields) + self._get_readonly_manual_pk_fields(obj)
 
     def get_prepopulated_fields(self, request, obj=None):
         """
@@ -2549,6 +2573,18 @@ def has_view_permission(self, request, obj=None):
             return self._has_any_perms_for_target_model(request, ["view", "change"])
         return super().has_view_permission(request)
 
+    def get_readonly_fields(self, request, obj=None):
+        """
+        Make manually specified (non-auto-created) primary key fields readonly
+        when editing an existing inline object.
+        """
+        readonly = list(self.readonly_fields)
+        if obj:
+            for pk in self.model._meta.pk_fields:
+                if pk.editable and not pk.auto_created and pk.name not in readonly:
+                    readonly.append(pk.name)
+        return readonly
+
 
 class StackedInline(InlineModelAdmin):
     template = "admin/edit_inline/stacked.html"
diff --git a/tests/admin_custom_urls/tests.py b/tests/admin_custom_urls/tests.py
index d401976ebb40..6eda92b33b71 100644
--- a/tests/admin_custom_urls/tests.py
+++ b/tests/admin_custom_urls/tests.py
@@ -95,7 +95,11 @@ def test_admin_URLs_no_clash(self):
         )
         response = self.client.get(url)
         self.assertContains(response, "Change action")
-        self.assertContains(response, 'value="path/to/html/document.html"')
+        self.assertContains(
+            response,
+            '<div class="readonly">path/to/html/document.html</div>',
+            html=True,
+        )
 
     def test_post_save_add_redirect(self):
         """
diff --git a/tests/admin_inlines/tests.py b/tests/admin_inlines/tests.py
index 0b7308995025..bc0978138875 100644
--- a/tests/admin_inlines/tests.py
+++ b/tests/admin_inlines/tests.py
@@ -19,6 +19,7 @@
     Child,
     ChildModel1,
     ChildModel2,
+    EditablePKBook,
     ExtraTerrestrial,
     Fashionista,
     FootNote,
@@ -708,6 +709,21 @@ def test_inline_editable_pk(self):
             count=1,
         )
 
+    def test_inline_manual_pk_is_readonly_when_editing(self):
+        author = Author.objects.create(name="Jane Austen")
+        EditablePKBook.objects.create(
+            author=author, manual_pk=101, title="Pride and Prejudice"
+        )
+
+        response = self.client.get(
+            reverse("admin:admin_inlines_author_change", args=[author.pk])
+        )
+
+        self.assertContains(
+            response, 'name="editablepkbook_set-0-manual_pk"', html=False
+        )
+        self.assertContains(response, "readonly", html=False)
+
     def test_stacked_inline_edit_form_contains_has_original_class(self):
         holder = Holder.objects.create(dummy=1)
         holder.inner_set.create(dummy=1)