diff --git a/core/modules/editor/src/Form/EditorImageDialog.php b/core/modules/editor/src/Form/EditorImageDialog.php
index 893b5a4f8065cb106b53f1b10d63e3884267a88e..23a4e203ae85a23c26bc92890ef00d09b056519a 100644
--- a/core/modules/editor/src/Form/EditorImageDialog.php
+++ b/core/modules/editor/src/Form/EditorImageDialog.php
@@ -96,6 +96,7 @@ public function buildForm(array $form, array &$form_state, FilterFormat $filter_
     $form['attributes']['alt'] = array(
       '#title' => $this->t('Alternative text'),
       '#type' => 'textfield',
+      '#required' => TRUE,
       '#default_value' => isset($image_element['alt']) ? $image_element['alt'] : '',
       '#maxlength' => 2048,
     );