diff -u b/core/includes/form.inc b/core/includes/form.inc
--- b/core/includes/form.inc
+++ b/core/includes/form.inc
@@ -532,10 +532,12 @@
 function template_preprocess_textarea(&$variables) {
   $element = $variables['element'];
   Element::setAttributes($element, array('id', 'name', 'rows', 'cols', 'placeholder'));
+  Element\RenderElement::setAttributes($element, array('form-textarea'));
   $variables['wrapper_attributes'] = new Attribute();
   $variables['attributes'] = new Attribute($element['#attributes']);
   $variables['value'] = String::checkPlain($element['#value']);
   $variables['resizable'] = !empty($element['#resizable']) ? $element['#resizable'] : NULL;
+  $variables['required'] = !empty($element['#required']) ? $element['#required'] : NULL;
 }
 
 /**
diff -u b/core/modules/system/templates/textarea.html.twig b/core/modules/system/templates/textarea.html.twig
--- b/core/modules/system/templates/textarea.html.twig
+++ b/core/modules/system/templates/textarea.html.twig
@@ -7,6 +7,7 @@
  * - wrapper_attributes: A list of HTML attributes for the wrapper element.
  * - attributes: A list of HTML attributes for the textarea element.
  * - resizable: Resizable setting value.
+ * - required: Required setting value.
  * - value: The textarea content.
  *
  * @see template_preprocess_textarea()
@@ -18,6 +19,7 @@
   set classes = [
     'form-textarea',
     resizable ? 'resize-' ~ resizable,
+    required ? 'required',
   ]
 %}
 <div{{ wrapper_attributes.addClass('form-textarea-wrapper') }}>