diff --git a/core/modules/block/lib/Drupal/block/BlockFormController.php b/core/modules/block/lib/Drupal/block/BlockFormController.php
index f36d8c1..7b55d62 100644
--- a/core/modules/block/lib/Drupal/block/BlockFormController.php
+++ b/core/modules/block/lib/Drupal/block/BlockFormController.php
@@ -248,6 +248,10 @@ public function form(array $form, array &$form_state) {
         '#options' => $theme_options,
         '#title' => t('Theme'),
         '#default_value' => $theme,
+        '#ajax' => array(
+          'callback' => array($this, 'themeSwitch'),
+          'wrapper' => 'edit-block-region-wrapper',
+         ),
       );
     }
     // Region settings.
@@ -258,11 +262,21 @@ public function form(array $form, array &$form_state) {
       '#default_value' => $entity->get('region'),
       '#empty_value' => BLOCK_REGION_NONE,
       '#options' => system_region_list($theme, REGIONS_VISIBLE),
+      '#prefix' => '<div id="edit-block-region-wrapper">',
+      '#suffix' => '</div>',
     );
     return $form;
   }
 
   /**
+   * Handles switching the available regions based on the selected theme.
+   */
+  public function themeSwitch($form, &$form_state) {
+    $form['region']['#options'] = system_region_list($form_state['values']['theme'], REGIONS_VISIBLE);
+    return $form['region'];
+  }
+
+  /**
    * {@inheritdoc}
    */
   protected function actions(array $form, array &$form_state) {