diff --git a/core/lib/Drupal/Core/EventSubscriber/MaintenanceModeSubscriber.php b/core/lib/Drupal/Core/EventSubscriber/MaintenanceModeSubscriber.php
index 23a2daa..6d48a3e 100644
--- a/core/lib/Drupal/Core/EventSubscriber/MaintenanceModeSubscriber.php
+++ b/core/lib/Drupal/Core/EventSubscriber/MaintenanceModeSubscriber.php
@@ -123,15 +123,23 @@ public function onKernelRequestMaintenance(RequestEvent $event) {
         $this->eventDispatcher->dispatch($event, MaintenanceModeEvents::MAINTENANCE_MODE_REQUEST);
       }
       else {
-        // Display a message if the logged in user has access to the site in
-        // maintenance mode. However, suppress it on the maintenance mode
-        // settings page.
+        // Display a message if the logged-in user has access to the site in
+        // maintenance mode. Don't show maintenance message:
+        // - on AJAX requests.
+        // - on Iframe uploads.
+        // - on the maintenance mode settings page.
         if ($route_match->getRouteName() != 'system.site_maintenance_mode') {
-          if ($this->account->hasPermission('administer site configuration')) {
-            $this->messenger->addMessage($this->t('Operating in maintenance mode. <a href=":url">Go online.</a>', [':url' => $this->urlGenerator->generate('system.site_maintenance_mode')]), 'status', FALSE);
-          }
-          else {
-            $this->messenger->addMessage($this->t('Operating in maintenance mode.'), 'status', FALSE);
+          $show_message = $route_match->getRouteName() != 'system.site_maintenance_mode' &&
+            !$event->getRequest()->isXmlHttpRequest() &&
+            $event->getRequest()->get('ajax_iframe_upload', FALSE) === FALSE;
+
+          if ($show_message) {
+            if ($this->account->hasPermission('administer site configuration')) {
+              $this->messenger->addMessage($this->t('Operating in maintenance mode. <a href=":url">Go online.</a>', [':url' => $this->urlGenerator->generate('system.site_maintenance_mode')]), 'status', FALSE);
+            }
+            else {
+              $this->messenger->addMessage($this->t('Operating in maintenance mode.'), 'status', FALSE);
+            }
           }
         }
       }
diff --git a/core/tests/Drupal/FunctionalJavascriptTests/Ajax/AjaxMaintenanceModeTest.php b/core/tests/Drupal/FunctionalJavascriptTests/Ajax/AjaxMaintenanceModeTest.php
new file mode 100644
index 0000000..4d4d72b
--- /dev/null
+++ b/core/tests/Drupal/FunctionalJavascriptTests/Ajax/AjaxMaintenanceModeTest.php
@@ -0,0 +1,67 @@
+<?php
+
+namespace Drupal\FunctionalJavascriptTests\Ajax;
+
+use Drupal\FunctionalJavascriptTests\WebDriverTestBase;
+use Drupal\Tests\field_ui\Traits\FieldUiTestTrait;
+use Drupal\Tests\file\Functional\FileFieldCreationTrait;
+use Drupal\Tests\TestFileCreationTrait;
+
+/**
+ * Tests maintenance message during an AJAX call.
+ *
+ * @group Ajax
+ */
+class AjaxMaintenanceModeTest extends WebDriverTestBase {
+
+  use FieldUiTestTrait;
+  use FileFieldCreationTrait;
+  use TestFileCreationTrait;
+
+  /**
+   * An user with administration permissions.
+   *
+   * @var \Drupal\user\UserInterface
+   */
+  protected $adminUser;
+
+  /**
+   * {@inheritdoc}
+   */
+  protected static $modules = ['ajax_test'];
+
+  /**
+   * {@inheritdoc}
+   */
+  protected $defaultTheme = 'stark';
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp(): void {
+    parent::setUp();
+    $this->adminUser = $this->drupalCreateUser([
+      'access administration pages',
+      'administer site configuration',
+      'access site in maintenance mode',
+    ]);
+    $this->drupalLogin($this->adminUser);
+  }
+
+  /**
+   * Tests maintenance message only appears once on an AJAX call.
+   */
+  public function testAjaxCallMaintenanceMode(): void {
+    $page = $this->getSession()->getPage();
+    $assert_session = $this->assertSession();
+
+    \Drupal::state()->set('system.maintenance_mode', TRUE);
+
+    $this->drupalGet('ajax-test/insert-inline-wrapper');
+    $assert_session->pageTextContains('Target inline');
+    $page->clickLink('Link html pre-wrapped-div');
+    $this->assertSession()->assertWaitOnAjaxRequest();
+    $this->assertSession()->pageTextContainsOnce('Operating in maintenance mode');
+  }
+
+}