diff --git a/core/lib/Drupal/Core/Render/Element/RenderElement.php b/core/lib/Drupal/Core/Render/Element/RenderElement.php
index 802e05f1c4..d4c7ab383c 100644
--- a/core/lib/Drupal/Core/Render/Element/RenderElement.php
+++ b/core/lib/Drupal/Core/Render/Element/RenderElement.php
@@ -330,6 +330,7 @@ public static function preRenderAjaxForm($element) {
 
       // @todo Remove support for 'wrapper' and solely use 'wrapper_selector'.
       if (isset($settings['wrapper'])) {
+        @trigger_error('Use of "wrapper" is deprecated in Drupal 8.7.x and will be removed before Drupal 9.0.0. Use "wrapper_selector" instead and prefix the value with either "#" for IDs or "." for classes.', E_USER_DEPRECATED);
         $settings['wrapper_selector'] = '#' . $settings['wrapper'];
         unset($settings['wrapper']);
       }
diff --git a/core/modules/action/tests/action_form_ajax_test/src/Plugin/Action/ActionAjaxTest.php b/core/modules/action/tests/action_form_ajax_test/src/Plugin/Action/ActionAjaxTest.php
index 8afc98cf54..02075a1d28 100644
--- a/core/modules/action/tests/action_form_ajax_test/src/Plugin/Action/ActionAjaxTest.php
+++ b/core/modules/action/tests/action_form_ajax_test/src/Plugin/Action/ActionAjaxTest.php
@@ -50,7 +50,7 @@ public function buildConfigurationForm(array $form, FormStateInterface $form_sta
       '#type' => 'checkbox',
       '#title' => $this->t('Are we having a party?'),
       '#ajax' => [
-        'wrapper' => 'party-container',
+        'wrapper_selector' => '#party-container',
         'callback' => [$this, 'partyCallback'],
       ],
       '#default_value' => $having_a_party,
diff --git a/core/modules/layout_builder/src/Plugin/Block/FieldBlock.php b/core/modules/layout_builder/src/Plugin/Block/FieldBlock.php
index 88e608f6f4..3f8bb32565 100644
--- a/core/modules/layout_builder/src/Plugin/Block/FieldBlock.php
+++ b/core/modules/layout_builder/src/Plugin/Block/FieldBlock.php
@@ -251,7 +251,7 @@ public function blockForm($form, FormStateInterface $form_state) {
       '#default_value' => $config['formatter']['type'],
       '#ajax' => [
         'callback' => [static::class, 'formatterSettingsAjaxCallback'],
-        'wrapper' => 'formatter-settings-wrapper',
+        'wrapper_selector' => '#formatter-settings-wrapper',
       ],
     ];
 
diff --git a/core/modules/layout_builder/tests/modules/layout_builder_test/src/Plugin/Block/TestAjaxBlock.php b/core/modules/layout_builder/tests/modules/layout_builder_test/src/Plugin/Block/TestAjaxBlock.php
index 1f1eeb0d7f..440921b0b3 100644
--- a/core/modules/layout_builder/tests/modules/layout_builder_test/src/Plugin/Block/TestAjaxBlock.php
+++ b/core/modules/layout_builder/tests/modules/layout_builder_test/src/Plugin/Block/TestAjaxBlock.php
@@ -32,7 +32,7 @@ public function blockForm($form, FormStateInterface $form_state) {
         '@time' => time(),
       ]),
       '#ajax' => [
-        'wrapper' => 'test-ajax-wrapper',
+        'wrapper_selector' => '#test-ajax-wrapper',
         'callback' => [$this, 'ajaxCallback'],
       ],
     ];
diff --git a/core/modules/media_library/src/Form/MediaLibraryUploadForm.php b/core/modules/media_library/src/Form/MediaLibraryUploadForm.php
index ac250b1077..9cca26106b 100644
--- a/core/modules/media_library/src/Form/MediaLibraryUploadForm.php
+++ b/core/modules/media_library/src/Form/MediaLibraryUploadForm.php
@@ -215,7 +215,7 @@ public function buildForm(array $form, FormStateInterface $form_state) {
             '#submit' => ['::selectType'],
             '#ajax' => [
               'callback' => '::updateFormCallback',
-              'wrapper' => 'media-library-upload-wrapper',
+              'wrapper_selector' => '#media-library-upload-wrapper',
             ],
             '#limit_validation_errors' => [['files', $i, $type->id()]],
           ];
@@ -230,7 +230,7 @@ public function buildForm(array $form, FormStateInterface $form_state) {
         '#value' => $this->t('Save'),
         '#ajax' => [
           'callback' => '::updateWidget',
-          'wrapper' => 'media-library-upload-wrapper',
+          'wrapper_selector' => '#media-library-upload-wrapper',
         ],
       ];
     }
@@ -338,7 +338,7 @@ public function processUploadElement(array $element, FormStateInterface $form_st
     $element['upload_button']['#submit'] = ['::uploadButtonSubmit'];
     $element['upload_button']['#ajax'] = [
       'callback' => '::updateFormCallback',
-      'wrapper' => 'media-library-upload-wrapper',
+      'wrapper_selector' => '#media-library-upload-wrapper',
     ];
     return $element;
   }
diff --git a/core/modules/media_library/src/Plugin/Field/FieldWidget/MediaLibraryWidget.php b/core/modules/media_library/src/Plugin/Field/FieldWidget/MediaLibraryWidget.php
index 22ac3ecb7a..c71e47e84f 100644
--- a/core/modules/media_library/src/Plugin/Field/FieldWidget/MediaLibraryWidget.php
+++ b/core/modules/media_library/src/Plugin/Field/FieldWidget/MediaLibraryWidget.php
@@ -192,7 +192,7 @@ public function formElement(FieldItemListInterface $items, $delta, array $elemen
             ],
             '#ajax' => [
               'callback' => [static::class, 'updateWidget'],
-              'wrapper' => $wrapper_id,
+              'wrapper_selector' => '#' . $wrapper_id,
             ],
             '#submit' => [[static::class, 'removeItem']],
             // Prevent errors in other widgets from preventing removal.
@@ -297,7 +297,7 @@ public function formElement(FieldItemListInterface $items, $delta, array $elemen
       '#name' => $field_name . '-media-library-update' . $id_suffix,
       '#ajax' => [
         'callback' => [static::class, 'updateWidget'],
-        'wrapper' => $wrapper_id,
+        'wrapper_selector' => '#' . $wrapper_id,
       ],
       '#attributes' => [
         'data-media-library-widget-update' => $field_name . $id_suffix,
diff --git a/core/modules/system/tests/modules/ajax_forms_test/src/Form/AjaxFormsTestAjaxElementsForm.php b/core/modules/system/tests/modules/ajax_forms_test/src/Form/AjaxFormsTestAjaxElementsForm.php
index 41a9b6d92a..c657399ea8 100644
--- a/core/modules/system/tests/modules/ajax_forms_test/src/Form/AjaxFormsTestAjaxElementsForm.php
+++ b/core/modules/system/tests/modules/ajax_forms_test/src/Form/AjaxFormsTestAjaxElementsForm.php
@@ -39,7 +39,7 @@ public function buildForm(array $form, FormStateInterface $form_state) {
       '#type' => 'datetime',
       '#ajax' => [
         'callback' => [$callback_object, 'datetimeCallback'],
-        'wrapper' => 'ajax_datetime_value',
+        'wrapper_selector' => '#ajax_datetime_value',
       ],
     ];
 
diff --git a/core/modules/system/tests/modules/form_test/src/Form/FormTestMachineNameValidationForm.php b/core/modules/system/tests/modules/form_test/src/Form/FormTestMachineNameValidationForm.php
index ff2dcaac58..4bf59dd3df 100644
--- a/core/modules/system/tests/modules/form_test/src/Form/FormTestMachineNameValidationForm.php
+++ b/core/modules/system/tests/modules/form_test/src/Form/FormTestMachineNameValidationForm.php
@@ -76,7 +76,7 @@ public function buildForm(array $form, FormStateInterface $form_state) {
       '#required' => TRUE,
       '#ajax' => [
         'callback' => '::buildAjaxSnackConfigureForm',
-        'wrapper' => 'snack-config-form',
+        'wrapper_selector' => '#snack-config-form',
         'method' => 'replace',
         'effect' => 'fade',
       ],
diff --git a/core/modules/system/tests/modules/js_ajax_test/src/Form/JsAjaxTestForm.php b/core/modules/system/tests/modules/js_ajax_test/src/Form/JsAjaxTestForm.php
index 567d4158d6..500a41458d 100644
--- a/core/modules/system/tests/modules/js_ajax_test/src/Form/JsAjaxTestForm.php
+++ b/core/modules/system/tests/modules/js_ajax_test/src/Form/JsAjaxTestForm.php
@@ -40,7 +40,7 @@ public function buildForm(array $form, FormStateInterface $form_state) {
           'type' => 'throbber',
           'message' => NULL,
         ],
-        'wrapper' => 'js_ajax_test_form_wrapper',
+        'wrapper_selector' => '#js_ajax_test_form_wrapper',
       ],
     ];
     return $form;