diff --git a/src/wp-includes/query.php b/src/wp-includes/query.php
index ba9ac64d44..5b36393ac7 100644
--- a/src/wp-includes/query.php
+++ b/src/wp-includes/query.php
@@ -4616,6 +4616,7 @@ class WP_Query {
 	 * Set up global post data.
 	 *
 	 * @since 4.1.0
+	 * @since 4.4.0 Added the ability to pass a post ID to `$post`.
 	 *
 	 * @global int             $id
 	 * @global WP_User         $authordata
@@ -4627,12 +4628,20 @@ class WP_Query {
 	 * @global int             $more
 	 * @global int             $numpages
 	 *
-	 * @param WP_Post $post Post data.
+	 * @param WP_Post|object|int $post WP_Post instance or Post ID/object.
 	 * @return true True when finished.
 	 */
 	public function setup_postdata( $post ) {
 		global $id, $authordata, $currentday, $currentmonth, $page, $pages, $multipage, $more, $numpages;
 
+		if ( ! ( $post instanceof WP_Post ) ) {
+			$post = get_post( $post );
+		}
+
+		if ( ! $post ) {
+			return;
+		}
+
 		$id = (int) $post->ID;
 
 		$authordata = get_userdata($post->post_author);
@@ -4701,7 +4710,7 @@ class WP_Query {
 	public function reset_postdata() {
 		if ( ! empty( $this->post ) ) {
 			$GLOBALS['post'] = $this->post;
-			setup_postdata( $this->post );
+			$this->setup_postdata( $this->post );
 		}
 	}
 }
@@ -4769,10 +4778,11 @@ function wp_old_slug_redirect() {
  * Set up global post data.
  *
  * @since 1.5.0
+ * @since 4.4.0 Added the ability to pass a post ID to `$post`.
  *
  * @global WP_Query $wp_query
  *
- * @param object $post Post data.
+ * @param WP_Post|object|int $post WP_Post instance or Post ID/object.
  * @return bool True when finished.
  */
 function setup_postdata( $post ) {
diff --git a/tests/phpunit/tests/query/setupPostdata.php b/tests/phpunit/tests/query/setupPostdata.php
index a3f1a1ab46..c242a4e480 100644
--- a/tests/phpunit/tests/query/setupPostdata.php
+++ b/tests/phpunit/tests/query/setupPostdata.php
@@ -46,6 +46,41 @@ class Tests_Query_SetupPostdata extends WP_UnitTestCase {
 		$this->assertSame( $p->ID, $GLOBALS['id'] );
 	}
 
+	/**
+	 * @ticket 30970
+	 */
+	public function test_setup_by_id() {
+		$p = $this->factory->post->create_and_get();
+		setup_postdata( $p->ID );
+
+		$this->assertSame( $p->ID, $GLOBALS['id'] );
+	}
+
+	/**
+	 * @ticket 30970
+	 */
+	public function test_setup_by_fake_post() {
+		$fake = new stdClass;
+		$fake->ID = 98765;
+		setup_postdata( $fake->ID );
+
+		// Fails because there's no post with this ID.
+		$this->assertNotSame( $fake->ID, $GLOBALS['id'] );
+	}
+
+	/**
+	 * @ticket 30970
+	 */
+	public function test_setup_by_postish_object() {
+		$p = $this->factory->post->create();
+
+		$post = new stdClass();
+		$post->ID = $p;
+		setup_postdata( $p );
+
+		$this->assertSame( $p, $GLOBALS['id'] );
+	}
+
 	public function test_authordata() {
 		$u = $this->factory->user->create_and_get();
 		$p = $this->factory->post->create_and_get( array(