Index: src/wp-includes/default-filters.php
===================================================================
--- src/wp-includes/default-filters.php	(revision 50232)
+++ src/wp-includes/default-filters.php	(working copy)
@@ -341,6 +341,9 @@
 add_action( 'atom_head', 'atom_site_icon' );
 add_action( 'rss2_head', 'rss2_site_icon' );
 
+// Feed Post Thumbnail.
+add_action( 'rss2_item', 'the_post_thumbnail_rss' );
+
 
 // WP Cron.
 if ( ! defined( 'DOING_CRON' ) ) {
Index: src/wp-includes/feed-rss2.php
===================================================================
--- src/wp-includes/feed-rss2.php	(revision 50232)
+++ src/wp-includes/feed-rss2.php	(working copy)
@@ -27,6 +27,7 @@
 	xmlns:atom="http://www.w3.org/2005/Atom"
 	xmlns:sy="http://purl.org/rss/1.0/modules/syndication/"
 	xmlns:slash="http://purl.org/rss/1.0/modules/slash/"
+	xmlns:media="http://search.yahoo.com/mrss/"
 	<?php
 	/**
 	 * Fires at the end of the RSS root to add namespaces.
Index: src/wp-includes/feed.php
===================================================================
--- src/wp-includes/feed.php	(revision 50232)
+++ src/wp-includes/feed.php	(working copy)
@@ -613,6 +613,44 @@
 }
 
 /**
+ * Displays post thumbnail in RSS2.
+ *
+ * @since x.x.x
+ */
+function the_post_thumbnail_rss() {
+	if ( ! has_post_thumbnail() ) {
+		return;
+	}
+
+	$image     = get_post( get_post_thumbnail_id() );
+	$full      = wp_get_attachment_image_src( $image->ID, 'full' );
+	$thumbnail = wp_get_attachment_image_src( $image->ID, 'thumbnail' );
+	?>
+	<media:content
+		url="<?php echo esc_url( $full[0] ); ?>"
+		type="<?php echo esc_attr( $image->post_mime_type ); ?>"
+		medium="image"
+		width="<?php echo absint( $full[1] ); ?>"
+		height="<?php echo absint( $full[2] ); ?>"
+	>
+		<media:title type="plain">
+			<![CDATA[<?php echo sanitize_text_field( $image->post_title ); ?>]]>
+		</media:title>
+		<media:thumbnail
+			url="<?php echo esc_url( $thumbnail[0] ); ?>"
+			width="<?php echo absint( $thumbnail[1] ); ?>"
+			height="<?php echo absint( $thumbnail[2] ); ?>" />
+		<?php if ( ! empty( $image->post_content ) ) : ?>
+			<media:description type="plain"><![CDATA[<?php echo wp_kses_post( $image->post_content ); ?>]]></media:description>
+		<?php endif; ?>
+		<?php if ( ! empty( $image->post_author ) ) : ?>
+			<media:credit role="author" scheme="urn:ebu"><?php echo esc_html( get_the_author_meta( 'display_name', $image->post_author ) ); ?></media:credit>
+		<?php endif; ?>
+	</media:content>
+	<?php
+}
+
+/**
  * Displays Site Icon in atom feeds.
  *
  * @since 4.3.0
Index: tests/phpunit/tests/feed/rss2.php
===================================================================
--- tests/phpunit/tests/feed/rss2.php	(revision 50232)
+++ tests/phpunit/tests/feed/rss2.php	(working copy)
@@ -35,6 +35,8 @@
 			)
 		);
 
+		$attachment_id = self::factory()->attachment->create_upload_object( DIR_TESTDATA . '/images/canola.jpg' );
+
 		// Set a predictable time for testing date archives.
 		self::$post_date = strtotime( '2003-05-27 10:07:53' );
 
@@ -57,6 +59,7 @@
 		// Assign a category to those posts.
 		foreach ( self::$posts as $post ) {
 			wp_set_object_terms( $post, self::$category->slug, 'category' );
+			set_post_thumbnail( $post, $attachment_id );
 		}
 	}
 
@@ -260,6 +263,10 @@
 			// Comment RSS.
 			$comment_rss = xml_find( $items[ $key ]['child'], 'wfw:commentRss' );
 			$this->assertSame( html_entity_decode( get_post_comments_feed_link( $post->ID ) ), $comment_rss[0]['content'] );
+
+			// Post Thumbnail.
+			$post_thumbnail = xml_find( $items[ $key ]['child'], 'media:content' );
+			$this->assertSame( get_the_post_thumbnail_url( $post ), $post_thumbnail[0]['attributes']['url'] );
 		}
 	}