diff --git a/src/wp-includes/post.php b/src/wp-includes/post.php
index c2b60a83e9..87aaeef190 100644
--- a/src/wp-includes/post.php
+++ b/src/wp-includes/post.php
@@ -3228,24 +3228,28 @@ function wp_set_post_terms( $post_id = 0, $tags = '', $taxonomy = 'post_tag', $a
  * @since 2.1.0
  *
  * @param int $post_ID Post ID.
- * @param array $post_categories Optional. List of categories.
+ * @param array|int $post_categories Optional. List of categories or ID of category.
+ * @param bool $append If true, don't delete existing categories, just add on. If false, replace the categories with the new categories.
  * @return bool|mixed
  */
-function wp_set_post_categories($post_ID = 0, $post_categories = array()) {
+function wp_set_post_categories( $post_ID = 0, $post_categories = array(), $append = false ) {
 	$post_ID = (int) $post_ID;
 	$post_type = get_post_type( $post_ID );
 	$post_status = get_post_status( $post_ID );
 	// If $post_categories isn't already an array, make it one:
-	if ( !is_array($post_categories) || empty($post_categories) ) {
-		if ( 'post' == $post_type && 'auto-draft' != $post_status )
+	$post_categories = (array) $post_categories;
+	if ( empty( $post_categories ) ) {
+		if ( 'post' == $post_type && 'auto-draft' != $post_status ) {
 			$post_categories = array( get_option('default_category') );
-		else
+			$append = false;
+		} else {
 			$post_categories = array();
+		}
 	} else if ( 1 == count($post_categories) && '' == reset($post_categories) ) {
 		return true;
 	}
 
-	return wp_set_post_terms($post_ID, $post_categories, 'category');
+	return wp_set_post_terms( $post_ID, $post_categories, 'category', $append );
 }
 
 /**
diff --git a/tests/phpunit/tests/term.php b/tests/phpunit/tests/term.php
index e32894fbe1..ff3fd8a541 100644
--- a/tests/phpunit/tests/term.php
+++ b/tests/phpunit/tests/term.php
@@ -466,4 +466,37 @@ class Tests_Term extends WP_UnitTestCase {
 		$this->assertEquals( $tag_id, $terms[0]->term_id );
 		$this->assertEquals( 'This description is even more amazing!', $terms[0]->description );
 	}
+
+	function test_wp_set_post_categories() {
+		$post_id = $this->factory->post->create();
+		$post = get_post( $post_id );
+
+		$this->assertInternalType( 'array', $post->post_category );
+		$this->assertEquals( 1, count( $post->post_category ) );
+		$this->assertEquals( get_option( 'default_category' ), $post->post_category[0] );
+		$term1 = wp_insert_term( 'Foo', 'category' );
+		$term2 = wp_insert_term( 'Bar', 'category' );
+		$term3 = wp_insert_term( 'Baz', 'category' );
+		wp_set_post_categories( $post_id, array( $term1['term_id'], $term2['term_id'] ) );
+		$this->assertEquals( 2, count( $post->post_category ) );
+		$this->assertEquals( array( $term2['term_id'], $term1['term_id'] ) , $post->post_category );
+
+		wp_set_post_categories( $post_id, $term3['term_id'], true );
+		$this->assertEquals( array( $term2['term_id'], $term3['term_id'], $term1['term_id'] ) , $post->post_category );
+
+		$term4 = wp_insert_term( 'Burrito', 'category' );
+		wp_set_post_categories( $post_id, $term4['term_id'] );
+		$this->assertEquals( array( $term4['term_id'] ), $post->post_category );
+
+		wp_set_post_categories( $post_id, array( $term1['term_id'], $term2['term_id'] ), true );
+		$this->assertEquals( array( $term2['term_id'], $term4['term_id'],$term1['term_id'] ), $post->post_category );
+
+		wp_set_post_categories( $post_id, array(), true );
+		$this->assertEquals( 1, count( $post->post_category ) );
+		$this->assertEquals( get_option( 'default_category' ), $post->post_category[0] );
+
+		wp_set_post_categories( $post_id, array() );
+		$this->assertEquals( 1, count( $post->post_category ) );
+		$this->assertEquals( get_option( 'default_category' ), $post->post_category[0] );
+	}
 }