diff --git a/src/wp-includes/theme.php b/src/wp-includes/theme.php
index f4dfaeedc2..2f7dc1fca7 100644
--- a/src/wp-includes/theme.php
+++ b/src/wp-includes/theme.php
@@ -1535,6 +1535,24 @@ function add_theme_support( $feature ) {
 		$args = array_slice( func_get_args(), 1 );
 
 	switch ( $feature ) {
+		case 'post-thumbnails':
+			// All post types are already supported.
+			if ( true === get_theme_support( 'post-thumbnails' ) ) {
+				return;
+			}
+
+			/*
+			 * Merge post types with any that already declared their support
+			 * for post thumbnails.
+			 */
+			if ( is_array( $args[0] ) && is_array( $_wp_theme_features['post-thumbnails'][0] ) ) {
+				$post_types = array_unique( array_merge( $args[0], $_wp_theme_features['post-thumbnails'][0] ) );
+
+				$args = array( $post_types );
+			}
+
+			break;
+
 		case 'post-formats' :
 			if ( is_array( $args[0] ) ) {
 				$post_formats = get_post_format_slugs();
diff --git a/tests/phpunit/tests/theme/support.php b/tests/phpunit/tests/theme/support.php
index c5669f8f5b..28180dcb93 100644
--- a/tests/phpunit/tests/theme/support.php
+++ b/tests/phpunit/tests/theme/support.php
@@ -52,6 +52,23 @@ class Tests_Theme_Support extends WP_UnitTestCase {
 		$this->assertFalse( current_theme_supports( 'post-thumbnails' ) );
 	}
 
+	/**
+	 * @ticket 22080
+	 */
+	public function test_post_thumbnails_mixed_args() {
+		add_theme_support( 'post-thumbnails', array( 'post', 'page' ) );
+		add_theme_support( 'post-thumbnails', array( 'page' ) );
+		$this->assertTrue( current_theme_support( 'post-thumbnails', 'post' ) );
+		$this->assertFalse( current_theme_supports( 'post-thumbnails', 'book' ) );
+		$this->assertEquals(
+			array( 0 => array( 'post', 'page' ) ),
+			get_theme_support( 'post-thumbnails' )
+		);
+
+		add_theme_support( 'post-thumbnails' );
+		$this->assertTrue( current_theme_supports( 'post-thumbnails', 'book' ) );
+	}
+
 	public function test_post_thumbnails_types_true() {
 		// array of arguments, with the key of 'types' holding the post types.
 		add_theme_support( 'post-thumbnails', array( 'types' => true ) );