diff --git a/changelogs/fragments/10075-keycloak_user_rolemapping-diff.yml b/changelogs/fragments/10075-keycloak_user_rolemapping-diff.yml
new file mode 100644
index 0000000000..926552ef9d
--- /dev/null
+++ b/changelogs/fragments/10075-keycloak_user_rolemapping-diff.yml
@@ -0,0 +1,2 @@
+bugfixes:
+  - "keycloak_user_rolemapping - fix ``--diff`` mode (https://github.com/ansible-collections/community.general/issues/10067, https://github.com/ansible-collections/community.general/pull/10075)."
diff --git a/plugins/modules/keycloak_user_rolemapping.py b/plugins/modules/keycloak_user_rolemapping.py
index c7af801706..c15bf09410 100644
--- a/plugins/modules/keycloak_user_rolemapping.py
+++ b/plugins/modules/keycloak_user_rolemapping.py
@@ -350,7 +350,7 @@ def main():
             # Assign roles
             result['changed'] = True
             if module._diff:
-                result['diff'] = dict(before=assigned_roles_before, after=update_roles)
+                result['diff'] = dict(before={"roles": assigned_roles_before}, after={"roles": update_roles})
             if module.check_mode:
                 module.exit_json(**result)
             kc.add_user_rolemapping(uid=uid, cid=cid, role_rep=update_roles, realm=realm)
@@ -365,7 +365,7 @@ def main():
             # Remove mapping of role
             result['changed'] = True
             if module._diff:
-                result['diff'] = dict(before=assigned_roles_before, after=update_roles)
+                result['diff'] = dict(before={"roles": assigned_roles_before}, after={"roles": update_roles})
             if module.check_mode:
                 module.exit_json(**result)
             kc.delete_user_rolemapping(uid=uid, cid=cid, role_rep=update_roles, realm=realm)