diff --git a/core/modules/field/src/Tests/FormTest.php b/core/modules/field/src/Tests/FormTest.php
index 786e367d41..d90ed811d6 100644
--- a/core/modules/field/src/Tests/FormTest.php
+++ b/core/modules/field/src/Tests/FormTest.php
@@ -634,4 +634,35 @@ public function testLabelOnMultiValueFields() {
     $this->assertEscaped("<script>alert('a configurable field');</script>");
   }
 
+  /**
+   * Tests alter hooks on widgets that handle multiple values.
+   *
+   * @todo Do we want to add these tests to testFieldFormMultipleWidget()
+   * instead of creating a separate test here? Or do we want to test the alter
+   * hooks on single-value widgets and multivalue widgets together and move some
+   * code from testFieldFormSingle() here?
+   */
+  public function testFieldFormMultipleWidgetAlter() {
+    // Create a field with fixed cardinality, configure the form to use a
+    // "multiple" widget.
+    $field_storage = $this->fieldStorageMultiple;
+    $field_name = $field_storage['field_name'];
+    $this->field['field_name'] = $field_name;
+    FieldStorageConfig::create($field_storage)->save();
+    FieldConfig::create($this->field)->save();
+    entity_get_form_display($this->field['entity_type'], $this->field['bundle'], 'default')
+      ->setComponent($field_name, [
+        'type' => 'test_field_widget_multiple',
+      ])
+      ->save();
+
+    // Display creation form. The text added to the child elements should appear
+    // exactly 4 times, but we just test that it appears more than once.
+    $this->drupalGet('entity_test/add');
+    $this->assertUniqueText(' From hook_field_widget_multiple_form_alter(): description on parent element.', 'Parent form element has text from hook_field_widget_multiple_form_alter()');
+    $this->assertNoUniqueText(' From hook_field_widget_multiple_form_alter(): description on child element.', 'Child form elements have text from hook_field_widget_multiple_form_alter()');
+    $this->assertUniqueText(' From hook_field_widget_multiple_WIDGET_TYPE_form_alter(): description on parent element.', 'Parent form element has text from hook_field_widget_multiple_WIDGET_TYPE_form_alter()');
+    $this->assertNoUniqueText(' From hook_field_widget_multiple_WIDGET_TYPE_form_alter(): description on child element.', 'Child form elements have text from hook_field_widget_multiple_WIDGET_TYPE_form_alter()');
+  }
+
 }
diff --git a/core/modules/field/tests/modules/field_test/field_test.module b/core/modules/field/tests/modules/field_test/field_test.module
index 15f8708adf..6616fef047 100644
--- a/core/modules/field/tests/modules/field_test/field_test.module
+++ b/core/modules/field/tests/modules/field_test/field_test.module
@@ -108,6 +108,45 @@ function field_test_field_widget_form_alter(&$element, FormStateInterface $form_
   }
 }
 
+/**
+ * Implements hook_field_widget_multiple_form_alter().
+ */
+function field_test_field_widget_multiple_form_alter(array &$elements, FormStateInterface $form_state, array $context) {
+  // Add a css class to widget form elements for all fields of type mytype.
+  $field_definition = $context['items']->getFieldDefinition();
+  debug($field_definition->getType());
+  if ($field_definition->getType() == 'mytype') {
+    // Be sure not to overwrite existing attributes.
+    $elements['#attributes']['class'][] = 'myclass';
+  }
+  if (empty($elements['#description'])) {
+    $elements['#description'] = '';
+  }
+  $elements['#description'] .= ' From hook_field_widget_multiple_form_alter(): description on parent element.';
+  foreach ($Element::children($elements) as $delta => $element) {
+    if (empty($element['#description'])) {
+      $elements[$delta]['#description'] = '';
+    }
+    $elements[$delta]['#description'] .= ' From hook_field_widget_multiple_form_alter(): description on child element.';
+  }
+}
+
+/**
+ * Implements hook_field_widget_multiple_WIDGET_TYPE_form_alter().
+ */
+function field_test_field_widget_multiple_test_field_widget_multiple_form_alter(array &$elements, FormStateInterface $form_state, array $context) {
+  if (empty($elements['#description'])) {
+    $elements['#description'] = '';
+  }
+  $elements['#description'] .= ' From hook_field_widget_multiple_WIDGET_TYPE_form_alter(): description on parent element.';
+  foreach ($Element::children($elements) as $delta => $element) {
+    if (empty($element['#description'])) {
+      $elements[$delta]['#description'] = '';
+    }
+    $elements[$delta]['#description'] .= ' From hook_field_widget_multiple_WIDGET_TYPE_form_alter(): description on child element.';
+  }
+}
+
 /**
  * Implements hook_query_TAG_alter() for tag 'efq_table_prefixing_test'.
  *