diff --git a/src/wp-includes/widgets/class-wp-widget-categories.php b/src/wp-includes/widgets/class-wp-widget-categories.php
index 4dfbdae1f3..ad27f39517 100644
--- a/src/wp-includes/widgets/class-wp-widget-categories.php
+++ b/src/wp-includes/widgets/class-wp-widget-categories.php
@@ -34,6 +34,8 @@ class WP_Widget_Categories extends WP_Widget {
 	 * Outputs the content for the current Categories widget instance.
 	 *
 	 * @since 2.8.0
+	 * @since 4.2.0 Creates a unique HTML ID for the `<select>` element
+	 *              if more than one instance is displayed on the page.
 	 *
 	 * @staticvar bool $first_dropdown
 	 *
diff --git a/src/wp-includes/widgets/class-wp-widget-recent-comments.php b/src/wp-includes/widgets/class-wp-widget-recent-comments.php
index a5f21919b0..01b8d5989a 100644
--- a/src/wp-includes/widgets/class-wp-widget-recent-comments.php
+++ b/src/wp-includes/widgets/class-wp-widget-recent-comments.php
@@ -66,12 +66,18 @@ class WP_Widget_Recent_Comments extends WP_Widget {
 	 * Outputs the content for the current Recent Comments widget instance.
 	 *
 	 * @since 2.8.0
+	 * @since 5.4.0 Creates a unique HTML ID for the `<ul>` element
+	 *              if more than one instance is displayed on the page.
+	 *
+	 * @staticvar bool $first_instance
 	 *
 	 * @param array $args     Display arguments including 'before_title', 'after_title',
 	 *                        'before_widget', and 'after_widget'.
 	 * @param array $instance Settings for the current Recent Comments widget instance.
 	 */
 	public function widget( $args, $instance ) {
+		static $first_instance = true;
+
 		if ( ! isset( $args['widget_id'] ) ) {
 			$args['widget_id'] = $this->id;
 		}
@@ -116,7 +122,10 @@ class WP_Widget_Recent_Comments extends WP_Widget {
 			$output .= $args['before_title'] . $title . $args['after_title'];
 		}
 
-		$output .= '<ul id="recentcomments">';
+		$recent_comments_id = ( $first_instance ) ? 'recentcomments' : "recentcomments-{$this->number}";
+		$first_instance     = false;
+
+		$output .= '<ul id="' . esc_attr( $recent_comments_id ) . '">';
 		if ( is_array( $comments ) && $comments ) {
 			// Prime cache for associated posts. (Prime post term cache if we need it for permalinks.)
 			$post_ids = array_unique( wp_list_pluck( $comments, 'comment_post_ID' ) );