diff --git a/core/modules/system/system.module b/core/modules/system/system.module
index 0d472e52512104eeebb0a31cbc4af0e0c81b6ec3..b3ea7647e37e6d565fb86c1d5c6dbc813193d7ef 100644
--- a/core/modules/system/system.module
+++ b/core/modules/system/system.module
@@ -2683,16 +2683,16 @@ function system_preprocess_block(&$variables) {
 }
 
 /**
- * Implements hook_preprocess_HOOK() for system-plugin-ui-form.tpl.php.
+ * Prepares variables for system plugin UI form templates.
  *
- * The $variables array contains the following arguments:
- * - $form
- */
+ * Default template: system-plugin-ui-form.html.twig.
+ *
+ * @param array $variables
+ *   An associative array containing:
+ *   - form: The plugin form elements.
+*/
 function template_preprocess_system_plugin_ui_form(&$variables) {
   drupal_add_css(drupal_get_path('module', 'system') . '/system.plugin.ui.css');
-  $variables['left'] = drupal_render($variables['form']['left']);
-  $variables['right'] = drupal_render($variables['form']['right']);
-  $variables['form_submit'] = drupal_render_children($variables['form']);
 }
 
 /**
diff --git a/core/modules/system/templates/system-plugin-ui-form.html.twig b/core/modules/system/templates/system-plugin-ui-form.html.twig
new file mode 100644
index 0000000000000000000000000000000000000000..415a6bf2f2cd69a84ed35929586a117bc26b2261
--- /dev/null
+++ b/core/modules/system/templates/system-plugin-ui-form.html.twig
@@ -0,0 +1,31 @@
+{#
+/**
+ * @file
+ * Default theme implementation to configure blocks.
+ *
+ * Available variables:
+ * - form: The form elements which contains:
+ *   - left: Form elements that appear in the left column.
+ *   - right: Form elements that appear in the right column.
+ *
+ * @see template_preprocess()
+ * @see template_preprocess_system_plugin_ui_form()
+ *
+ * @ingroup themeable
+ */
+#}
+<div id="block-library" class="container">
+  <div class="left-col">
+    <div class="inside">
+      {{ form.left }}
+    </div>
+  </div>
+  <div class="right-col">
+    <div class="inside">
+      {{ form.right }}
+    </div>
+  </div>
+  {% if form -%}
+    <div class="bottom-bar">{{ form }}</div>
+  {%- endif -%}
+</div>
diff --git a/core/modules/system/templates/system-plugin-ui-form.tpl.php b/core/modules/system/templates/system-plugin-ui-form.tpl.php
deleted file mode 100644
index 8179076ce2d7f61692be16ab97616ccaeb11518f..0000000000000000000000000000000000000000
--- a/core/modules/system/templates/system-plugin-ui-form.tpl.php
+++ /dev/null
@@ -1,32 +0,0 @@
-<?php
-
-/**
- * @file
- * Default theme implementation to configure blocks.
- *
- * Available variables:
- * - $left: Any form array elements that should appear in the left hand column.
- * - $right: Any form array elements that should appear in the right hand column.
- * - $form_submit: Form submit button.
- *
- * @see template_preprocess_block_library_form()
- * @see theme_block_library_form()
- *
- * @ingroup themeable
- */
-?>
-<div id="block-library" class="container">
-  <div class="left-col">
-    <div class="inside">
-      <?php print $left; ?>
-    </div>
-  </div>
-  <div class="right-col">
-    <div class="inside">
-      <?php print $right; ?>
-    </div>
-  </div>
-  <?php if ($form_submit) { ?>
-  <div class="bottom-bar"><?php print $form_submit; ?></div>
-  <?php } ?>
-</div>