diff --git a/core/lib/Drupal/Core/Field/WidgetBase.php b/core/lib/Drupal/Core/Field/WidgetBase.php
index b1579f2445..b399166df7 100644
--- a/core/lib/Drupal/Core/Field/WidgetBase.php
+++ b/core/lib/Drupal/Core/Field/WidgetBase.php
@@ -241,8 +241,21 @@ protected function formMultipleElements(FieldItemListInterface $items, array &$f
           ],
         ];
       }
-    }
 
+      // Allow modules to alter the field multiple widget form element.
+      $context = [
+        'form' => $form,
+        'widget' => $this,
+        'items' => $items,
+        'delta' => $delta,
+        'default' => $this->isDefaultValueWidget($form_state),
+        'cardinality' => $cardinality,
+      ];
+      \Drupal::moduleHandler()->alter([
+         'field_widget_multiple_form',
+         'field_widget_multiple_' . $this->getPluginId() . '_form',
+      ], $elements, $form_state, $context);
+    }
     return $elements;
   }
 
@@ -311,12 +324,14 @@ public static function addMoreAjax(array $form, FormStateInterface $form_state)
    * Generates the form element for a single copy of the widget.
    */
   protected function formSingleElement(FieldItemListInterface $items, $delta, array $element, array &$form, FormStateInterface $form_state) {
+      $cardinality = $this->fieldDefinition->getFieldStorageDefinition()->getCardinality();
     $element += [
       '#field_parents' => $form['#parents'],
       // Only the first widget should be required.
       '#required' => $delta == 0 && $this->fieldDefinition->isRequired(),
       '#delta' => $delta,
       '#weight' => $delta,
+      '#cardinality' => $cardinality,
     ];
 
     $element = $this->formElement($items, $delta, $element, $form, $form_state);
@@ -329,6 +344,7 @@ protected function formSingleElement(FieldItemListInterface $items, $delta, arra
         'items' => $items,
         'delta' => $delta,
         'default' => $this->isDefaultValueWidget($form_state),
+        'cardinality' => $cardinality,
       ];
       \Drupal::moduleHandler()->alter(['field_widget_form', 'field_widget_' . $this->getPluginId() . '_form'], $element, $form_state, $context);
     }
diff --git a/core/modules/field/field.api.php b/core/modules/field/field.api.php
index 130f923180..1fd8427e8c 100644
--- a/core/modules/field/field.api.php
+++ b/core/modules/field/field.api.php
@@ -183,6 +183,7 @@ function hook_field_widget_info_alter(array &$info) {
  * @see \Drupal\Core\Field\WidgetBaseInterface::form()
  * @see \Drupal\Core\Field\WidgetBase::formSingleElement()
  * @see hook_field_widget_WIDGET_TYPE_form_alter()
+ * @see hook_field_widget_multiple_form_alter()
  */
 function hook_field_widget_form_alter(&$element, \Drupal\Core\Form\FormStateInterface $form_state, $context) {
   // Add a css class to widget form elements for all fields of type mytype.
@@ -212,6 +213,7 @@ function hook_field_widget_form_alter(&$element, \Drupal\Core\Form\FormStateInte
  * @see \Drupal\Core\Field\WidgetBaseInterface::form()
  * @see \Drupal\Core\Field\WidgetBase::formSingleElement()
  * @see hook_field_widget_form_alter()
+ * @see hook_field_widget_multiple_WIDGET_TYPE_form_alter()
  */
 function hook_field_widget_WIDGET_TYPE_form_alter(&$element, \Drupal\Core\Form\FormStateInterface $form_state, $context) {
   // Code here will only act on widgets of type WIDGET_TYPE.  For example,
@@ -220,6 +222,77 @@ function hook_field_widget_WIDGET_TYPE_form_alter(&$element, \Drupal\Core\Form\F
   $element['#autocomplete_route_name'] = 'mymodule.autocomplete_route';
 }
 
+/**
+ * Alter forms for multivalue field widgets provided by other modules.
+ *
+ * To adjust the individual widgets for the values themselves, use
+ * hook_field_widget_form_alter() instead.
+ *
+ * @param array $elements
+ *   The field widget form elements as constructed by
+ *   \Drupal\Core\Field\WidgetBaseInterface::formMultipleElements().
+ * @param \Drupal\Core\Form\FormStateInterface $form_state
+ *   The current state of the form.
+ * @param array $context
+ *   An associative array containing the following key-value pairs:
+ *   - form: The form structure to which widgets are being attached. This may be
+ *     a full form structure, or a sub-element of a larger form.
+ *   - widget: The widget plugin instance.
+ *   - items: The field values, as a
+ *     \Drupal\Core\Field\FieldItemListInterface object.
+ *   - delta: The order of this item in the array of subelements (0, 1, 2, etc).
+ *   - default: A boolean indicating whether the form is being shown as a dummy
+ *     form to set default values.
+ *
+ * @see \Drupal\Core\Field\WidgetBaseInterface::form()
+ * @see \Drupal\Core\Field\WidgetBase::formSingleElement()
+ * @see hook_field_widget_form_alter()
+ * @see hook_field_widget_multiple_WIDGET_TYPE_form_alter()
+ */
+function hook_field_widget_multiple_form_alter(array &$elements, \Drupal\Core\Form\FormStateInterface $form_state, array $context) {
+  // Add a css class to the overall element of any widget.
+  $elements['#attributes']['class'][] = 'myclass';
+}
+
+/**
+ * Alter multi-value widget forms for a widget provided by another module.
+ *
+ * Modules can implement hook_field_widget_multiple_WIDGET_TYPE_form_alter() to
+ * modify a specific widget form, rather than using
+ * hook_field_widget_form_alter() and checking the widget type.
+ *
+ * To modify the widget for individual items, use
+ * hook_field_widget_WIDGET_TYPE_form_alter() instead.
+ *
+ * @param array $elements
+ *   The field widget form elements as constructed by
+ *   \Drupal\Core\Field\WidgetBaseInterface::formMultipleElements().
+ * @param \Drupal\Core\Form\FormStateInterface $form_state
+ *   The current state of the form.
+ * @param array $context
+ *   An associative array. See hook_field_widget_form_alter() for the structure
+ *   and content of the array.
+ *
+ * @see \Drupal\Core\Field\WidgetBaseInterface::form()
+ * @see \Drupal\Core\Field\WidgetBase::formSingleElement()
+ * @see hook_field_widget_multiple_form_alter()
+ */
+function hook_field_widget_multiple_WIDGET_TYPE_form_alter(array &$elements, \Drupal\Core\Form\FormStateInterface $form_state, array $context) {
+  // Alter the overall title for the multivalue widget (the table header,
+  // above the rows).
+  $elements['#title'] = t('Review your %title entries', ['%title' => $elements['#title']]);
+
+  if (!empty($elements['#description'])) {
+    // Add a paragraph to the description, above all the rows.
+    $elements['#description'] = [
+      '#markup' => $elements['#description'],
+    ];
+  }
+  else {
+    $elements['#description'] = t('These items might be out of date.');
+  }
+}
+
 /**
  * @} End of "defgroup field_widget".
  */
diff --git a/core/modules/media/media.module b/core/modules/media/media.module
index 296945973a..420e81e4fd 100644
--- a/core/modules/media/media.module
+++ b/core/modules/media/media.module
@@ -9,6 +9,7 @@
 use Drupal\Core\Entity\EntityInterface;
 use Drupal\Core\Form\FormStateInterface;
 use Drupal\Core\Render\Element;
+use Drupal\Core\Render\Markup;
 use Drupal\Core\Routing\RouteMatchInterface;
 use Drupal\Core\Session\AccountInterface;
 use Drupal\Core\Url;
@@ -66,6 +67,10 @@ function media_theme() {
     'media' => [
       'render element' => 'elements',
     ],
+    'media_reference_help' => [
+      'render element' => 'element',
+      'base hook' => 'field_multiple_value_form',
+    ],
   ];
 }
 
@@ -172,3 +177,42 @@ function media_form_field_ui_field_storage_add_form_alter(&$form, FormStateInter
   $form['add']['new_storage_type']['#weight'] = 0;
   $form['add']['description_wrapper']['#weight'] = 1;
 }
+
+/**
+ * Implements hook_field_widget_multiple_WIDGET_TYPE_form_alter().
+ */
+function media_field_widget_multiple_entity_reference_autocomplete_form_alter(array &$elements, FormStateInterface $form_state, array $context) {
+  // Add a description to inform users where they can find and add new media.
+  // Only add the description on autocomplete widgets for entity reference
+  // field pointing to media.
+  if (!empty($elements[0]['target_id']['#type']) && ($elements[0]['target_id']['#type'] === 'entity_autocomplete') && ($elements[0]['target_id']['#target_type'] === 'media')) {
+    $elements['#theme'] = 'media_reference_help';
+
+    // Customize the label for the field widget.
+    $label = $elements['#title'];
+    $elements['#original_label'] = $label;
+    $use_existing_label = t('Use existing %title', ['%title' => $label]);
+
+    if ($context['cardinality'] === 1) {
+      $elements[0]['target_id']['#title'] = $use_existing_label;
+    }
+    else {
+      $elements['#title'] = $use_existing_label;
+    }
+  }
+}
+
+/**
+ * Implements hook_preprocess_HOOK() for media reference widgets.
+ */
+function media_preprocess_media_reference_help(&$variables) {
+  $element = $variables['element'];
+  // Use the user-defined label for the field to indicate that the form allows
+  // reusing existing media.
+  $variables['label'] = $element['#original_label'];
+  if (!empty($element[0]['target_id']['#selection_settings']['target_bundles'])) {
+    $variables['types'] = implode(", ", $element[0]['target_id']['#selection_settings']['target_bundles']);
+    $variables['media_add_page'] = Url::fromRoute('entity.media.add_page')->toString();
+    $variables['media_list'] = Url::fromRoute('entity.media.collection')->toString();
+  }
+}
diff --git a/core/modules/media/templates/media-reference-help.html.twig b/core/modules/media/templates/media-reference-help.html.twig
new file mode 100644
index 0000000000..d99ca4032c
--- /dev/null
+++ b/core/modules/media/templates/media-reference-help.html.twig
@@ -0,0 +1,50 @@
+{#
+/**
+ * @file
+ * Theme override for media reference fields.
+ *
+ * @todo I have no idea what I'm doing.
+ *
+ * @see template_preprocess_field_multiple_value_form()
+ */
+#}
+<fieldset>
+  <legend>{{ label }}</legend>
+  <h4>
+    {% trans %}
+      Create new {{ label|placeholder }}
+    {% endtrans %}
+  </h4>
+  {% trans %}
+    Create your media on the <a href="{{ media_add_page }}" target="_blank">media add page</a> (opens a new window), then add it by name to the field below.
+  {% endtrans %}
+
+  {% if multiple %}
+    <div class="js-form-item form-item">
+      {{ table }}
+  {% else %}
+    {% for element in elements %}
+      {{ element }}
+    {% endfor %}
+  {% endif %}
+
+  <div{{ description.attributes.addClass('description') }} >
+    {% trans %}
+      Type part of the media name. See the <a href="{{ media_list }}" target="_blank">media list</a> (opens a new window) to help locate media. Allowed media types: {{ types|placeholder }}
+    {% endtrans %}
+
+    {% if multiple %}
+      {% if description.content %}
+        {{ description.content }}
+      {% endif %}
+      {% if button %}
+        <div class="clearfix">{{ button }}</div>
+      {% endif %}
+    {% endif %}
+
+  </div>
+
+  {% if multiple %}
+    </div>
+  {% endif %}
+</fieldset>
diff --git a/core/modules/media/tests/src/FunctionalJavascript/MediaReferenceFieldHelpTest.php b/core/modules/media/tests/src/FunctionalJavascript/MediaReferenceFieldHelpTest.php
index 7d22484238..e407779a90 100644
--- a/core/modules/media/tests/src/FunctionalJavascript/MediaReferenceFieldHelpTest.php
+++ b/core/modules/media/tests/src/FunctionalJavascript/MediaReferenceFieldHelpTest.php
@@ -2,7 +2,13 @@
 
 namespace Drupal\Tests\media\FunctionalJavascript;
 
+use Behat\Mink\Element\ElementInterface;
 use Drupal\Component\Utility\Html;
+use Drupal\Core\Field\FieldStorageDefinitionInterface;
+use Drupal\Core\Url;
+use Drupal\field\Entity\FieldConfig;
+use Drupal\field\Entity\FieldStorageConfig;
+use Drupal\FunctionalJavascriptTests\DrupalSelenium2Driver;
 
 /**
  * Tests related to media reference fields.
@@ -11,6 +17,15 @@
  */
 class MediaReferenceFieldHelpTest extends MediaJavascriptTestBase {
 
+  protected $minkDefaultDriverClass = DrupalSelenium2Driver::class;
+
+  /**
+   * IDs of media types created during the tests.
+   *
+   * @var string[]
+   */
+  protected $mediaTypes = [];
+
   /**
    * Test our custom help texts when creating a field.
    *
@@ -53,4 +68,166 @@ public function testFieldCreationHelpText() {
     }
   }
 
+  /**
+   * Test our custom help texts when creating a field.
+   *
+   * @see media_field_widget_entity_reference_autocomplete_form_alter()
+   * @see media_field_widget_multiple_entity_reference_autocomplete_form_alter()
+   */
+  public function testMediaAutocompleteWidgetTest() {
+    $assert_session = $this->assertSession();
+
+    $this->mediaTypes = [
+      $this->createMediaType()->id(),
+      $this->createMediaType()->id(),
+    ];
+
+    $content_type = $this->createContentType();
+    // The first field will have cardinality 1.
+    $storage1 = FieldStorageConfig::create([
+      'field_name' => 'field_card_1',
+      'entity_type' => 'node',
+      'type' => 'entity_reference',
+      'cardinality' => 1,
+      'settings' => [
+        'target_type' => 'media',
+      ],
+    ]);
+    $storage1->save();
+    $field1 = FieldConfig::create([
+      'label' => 'Media (card 1)',
+      'field_storage' => $storage1,
+      'entity_type' => 'node',
+      'bundle' => $content_type->id(),
+      'settings' => [
+        'handler' => 'default',
+        'handler_settings' => [
+          'target_bundles' => array_combine($this->mediaTypes, $this->mediaTypes),
+        ],
+      ],
+    ]);
+    $field1->save();
+    entity_get_form_display('node', $content_type->id(), 'default')
+      ->setComponent('field_card_1', [
+        'type' => 'entity_reference_autocomplete',
+      ])
+      ->save();
+
+    // Do the same with a field with unlimited cardinality.
+    $storage2 = FieldStorageConfig::create([
+      'field_name' => 'field_card_unlimited',
+      'entity_type' => 'node',
+      'type' => 'entity_reference',
+      'cardinality' => FieldStorageDefinitionInterface::CARDINALITY_UNLIMITED,
+      'settings' => [
+        'target_type' => 'media',
+      ],
+    ]);
+    $storage2->save();
+    $field2 = FieldConfig::create([
+      'label' => 'Media (card unlimited)',
+      'field_storage' => $storage2,
+      'entity_type' => 'node',
+      'bundle' => $content_type->id(),
+      'settings' => [
+        'handler' => 'default',
+        'handler_settings' => [
+          'target_bundles' => array_combine($this->mediaTypes, $this->mediaTypes),
+        ],
+      ],
+    ]);
+    $field2->save();
+    entity_get_form_display('node', $content_type->id(), 'default')
+      ->setComponent('field_card_unlimited', [
+        'type' => 'entity_reference_autocomplete',
+      ])
+      ->save();
+
+    // Check that both widgets display the expected help text on the form.
+    $this->drupalGet("/node/add/{$content_type->id()}");
+
+    // Check that the single-cardinality field is present and contains the stuff
+    // we expect.
+    $single_cardinality = $assert_session->elementExists('css', '#edit-field-card-1-wrapper fieldset');
+    $this->assertSame($field1->getLabel(), $assert_session->elementExists('css', 'legend', $single_cardinality)->getText());
+    $h4 = $assert_session->elementExists('css', 'h4', $single_cardinality);
+    $this->assertSame("Create new {$field1->getLabel()}", $h4->getText());
+    $this->assertHelpText($single_cardinality);
+
+    // Check that the unlimited-cardinality field is present and contains the
+    // stuff we expect.
+    /** @var \Behat\Mink\Element\ElementInterface $unlimited_cardinality */
+    $unlimited_cardinality = $assert_session->elementExists('css', '#edit-field-card-unlimited-wrapper fieldset');
+    $this->assertSame($field2->getLabel(), $assert_session->elementExists('css', 'legend', $unlimited_cardinality)->getText());
+    $h4s = $unlimited_cardinality->findAll('css', 'h4');
+    $this->assertCount(2, $h4s);
+    $this->assertSame("Create new {$field2->getLabel()}", $h4s[0]->getText());
+    // The multiple-widget has an empty h4 in our way, that's why we use the
+    // next one instead.
+    $this->assertEquals("Use existing {$field2->getLabel()}", $h4s[1]->getText());
+    $this->assertHelpText($unlimited_cardinality);
+
+    // Create a non-media entity reference and make sure our help text is not
+    // showing up.
+    $storage3 = FieldStorageConfig::create([
+      'field_name' => 'field_not_a_media_field',
+      'entity_type' => 'node',
+      'type' => 'entity_reference',
+      'cardinality' => FieldStorageDefinitionInterface::CARDINALITY_UNLIMITED,
+      'settings' => [
+        'target_type' => 'node',
+      ],
+    ]);
+    $storage3->save();
+    $field3 = FieldConfig::create([
+      'label' => 'No media here!',
+      'field_storage' => $storage3,
+      'entity_type' => 'node',
+      'bundle' => $content_type->id(),
+      'settings' => [
+        'handler' => 'default',
+        'handler_settings' => [
+          'target_bundles' => [
+            $content_type->id() => $content_type->id(),
+          ],
+        ],
+      ],
+    ]);
+    $field3->save();
+    entity_get_form_display('node', $content_type->id(), 'default')
+      ->removeComponent('field_card_1')
+      ->removeComponent('field_card_unlimited')
+      ->setComponent('field_not_a_media_field', [
+        'type' => 'entity_reference_autocomplete',
+      ])
+      ->save();
+    $this->drupalGet("/node/add/{$content_type->id()}");
+    $assert_session->pageTextNotContains('Create your media on the media add page (opens a new window), then add it by name to the field below.');
+    $assert_session->pageTextNotContains('Type part of the media name. See the media list (opens a new window) to help locate media.');
+  }
+
+  /**
+   * Asserts that the expected help text is found in an element.
+   *
+   * @param \Behat\Mink\Element\ElementInterface $element
+   *   The element.
+   */
+  protected function assertHelpText(ElementInterface $element) {
+    $element_text = $element->getText();
+    $assert = $this->assertSession();
+
+    $help_text = "Create your media on the media add page (opens a new window), then add it by name to the field below.";
+    $this->assertContains($help_text, $element_text);
+
+    $link = $assert->elementExists('named', ['link', 'media add page'], $element);
+    $href = Url::fromRoute('entity.media.add_page')->toString();
+    $this->assertSame($href, $link->getAttribute('href'));
+
+    $description = "Type part of the media name. See the media list (opens a new window) to help locate media. Allowed media types: " . implode(', ', $this->mediaTypes);
+    $this->assertContains($description, $element_text);
+    $link = $assert->elementExists('named', ['link', 'media list']);
+    $href = Url::fromRoute('entity.media.collection')->toString();
+    $this->assertSame($href, $link->getAttribute('href'));
+  }
+
 }