diff --git a/wp-admin/includes/post.php b/wp-admin/includes/post.php
index 83756e695a..f5f6234994 100644
--- a/wp-admin/includes/post.php
+++ b/wp-admin/includes/post.php
@@ -1396,10 +1396,31 @@ function _wp_post_thumbnail_html( $thumbnail_id = null, $post = null ) {
 	if ( $thumbnail_id && get_post( $thumbnail_id ) ) {
 		$old_content_width = $content_width;
 		$content_width = 266;
-		if ( !isset( $_wp_additional_image_sizes['post-thumbnail'] ) )
-			$thumbnail_html = wp_get_attachment_image( $thumbnail_id, array( $content_width, $content_width ) );
-		else
-			$thumbnail_html = wp_get_attachment_image( $thumbnail_id, 'post-thumbnail' );
+
+		$size = isset( $_wp_additional_image_sizes['post-thumbnail'] ) ? 'post-thumbnail' : array( $content_width, $content_width );
+
+		/**
+		 * Filter the size used to display the post thumbnail image in the 'Featured Image' meta box.
+		 *
+		 * Note: When a theme adds 'post-thumbnail' support, a special 'post-thumbnail'
+		 * image size is registered, which differs from the 'thumbnail' image size
+		 * managed via the Settings > Media screen. See the `$size` parameter description
+		 * for more information on default values.
+		 *
+		 * @since 4.4.0
+		 *
+		 * @param string|array $size         Post thumbnail image size to display in the meta box. Accepts
+		 *                                   any valid image size, or an array of height and width values.
+		 *                                   If the 'post-thumbnail' size is set, default is 'post-thumbnail'.
+		 *                                   Otherwise, default is an array with `$content_width` as the
+		 *                                   height and width values.
+		 * @param int          $thumbnail_id Post thumbnail attachment ID.
+		 * @param WP_Post      $post         The post object associated with the thumbnail.
+		 */
+		$size = apply_filters( 'admin_post_thumbnail_size', $size, $thumbnail_id, $post );
+
+		$thumbnail_html = wp_get_attachment_image( $thumbnail_id, $size );
+
 		if ( !empty( $thumbnail_html ) ) {
 			$ajax_nonce = wp_create_nonce( 'set_post_thumbnail-' . $post->ID );
 			$content = sprintf( $set_thumbnail_link,
diff --git a/wp-includes/version.php b/wp-includes/version.php
index 60d541f04c..59e59e31c1 100644
--- a/wp-includes/version.php
+++ b/wp-includes/version.php
@@ -4,7 +4,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '4.4-alpha-35021';
+$wp_version = '4.4-alpha-35022';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.