diff --git a/src/wp-includes/post.php b/src/wp-includes/post.php
index 5ebf42f89d..33a4c43314 100644
--- a/src/wp-includes/post.php
+++ b/src/wp-includes/post.php
@@ -6251,16 +6251,15 @@ function _publish_post_hook( $post_id ) {
 }
 
 /**
- * Return the post's parent's post_ID
+ * Return the post's parent post ID.
  *
  * @since 3.1.0
  *
- * @param int $post_ID
- *
+ * @param int|WP_Post $post Post ID or post object. Defaults to global $post.
  * @return int|false Post parent ID, otherwise false.
  */
-function wp_get_post_parent_id( $post_ID ) {
-	$post = get_post( $post_ID );
+function wp_get_post_parent_id( $post ) {
+	$post = get_post( $post );
 	if ( ! $post || is_wp_error( $post ) ) {
 		return false;
 	}
diff --git a/tests/phpunit/tests/post/wpGetPostParentId.php b/tests/phpunit/tests/post/wpGetPostParentId.php
new file mode 100644
index 0000000000..1f54934d9b
--- /dev/null
+++ b/tests/phpunit/tests/post/wpGetPostParentId.php
@@ -0,0 +1,42 @@
+<?php
+
+/**
+ * @group post
+ */
+class Tests_Post_WpGetPostParentId extends WP_UnitTestCase {
+
+	public function test_wp_get_post_parent_id_with_post_object() {
+		$p1 = self::factory()->post->create();
+		$p2 = self::factory()->post->create( array( 'post_parent' => $p1 ) );
+		$post = get_post( $p2 );
+		$this->assertTrue( $post instanceof WP_Post );
+		$this->assertEquals( $p1, wp_get_post_parent_id( $post ) );
+	}
+
+	public function test_wp_get_post_parent_id_with_post_id() {
+		$p1 = self::factory()->post->create();
+		$p2 = self::factory()->post->create( array( 'post_parent' => $p1 ) );
+		$this->assertEquals( $p1, wp_get_post_parent_id( $p2 ) );
+	}
+
+	public function test_wp_get_post_parent_id_with_non_existing_id_default_to_global_post_id() {
+		$p1 = self::factory()->post->create();
+		$GLOBALS['post'] = self::factory()->post->create( array( 'post_parent' => $p1 ) );
+		$this->assertEquals( $p1, wp_get_post_parent_id( 0 ) );
+		unset( $GLOBALS['post'] );
+	}
+
+	public function test_wp_get_post_parent_id_with_boolean_default_to_global_post_id() {
+		$p1 = self::factory()->post->create();
+		$GLOBALS['post'] = self::factory()->post->create( array( 'post_parent' => $p1 ) );
+		$this->assertEquals( $p1, wp_get_post_parent_id( false ) );
+		unset( $GLOBALS['post'] );
+	}
+
+	public function test_wp_get_post_parent_id_with_string_default_to_false() {
+		$p1 = self::factory()->post->create();
+		$GLOBALS['post'] = self::factory()->post->create( array( 'post_parent' => $p1 ) );
+		$this->assertFalse( wp_get_post_parent_id( 'string' ) );
+		unset( $GLOBALS['post'] );
+	}
+}