diff --git src/wp-includes/class-wp-query.php src/wp-includes/class-wp-query.php
index e569e02ef1..0f38c6479b 100644
--- src/wp-includes/class-wp-query.php
+++ src/wp-includes/class-wp-query.php
@@ -1546,6 +1546,9 @@ class WP_Query {
 			'menu_order',
 			'comment_count',
 			'rand',
+			'post__in',
+			'post_parent__in',
+			'post_name__in',
 		);
 
 		$primary_meta_key   = '';
@@ -1576,6 +1579,8 @@ class WP_Query {
 			return false;
 		}
 
+		$orderby_clause = '';
+
 		switch ( $orderby ) {
 			case 'post_name':
 			case 'post_author':
@@ -1603,6 +1608,23 @@ class WP_Query {
 			case 'meta_value_num':
 				$orderby_clause = "{$primary_meta_query['alias']}.meta_value+0";
 				break;
+			case 'post__in':
+				if ( ! empty( $this->query_vars['post__in'] ) ) {
+					$orderby_clause = "FIELD({$wpdb->posts}.ID," . implode( ',', array_map( 'absint', $this->query_vars['post__in'] ) ) . ')';
+				}
+				break;
+			case 'post_parent__in':
+				if ( ! empty( $this->query_vars['post_parent__in'] ) ) {
+					$orderby_clause = "FIELD( {$wpdb->posts}.post_parent," . implode( ', ', array_map( 'absint', $this->query_vars['post_parent__in'] ) ) . ' )';
+				}
+				break;
+			case 'post_name__in':
+				if ( ! empty( $this->query_vars['post_name__in'] ) ) {
+					$post_name__in = array_map( 'sanitize_title_for_query', $this->query_vars['post_name__in'] );
+					$post_name__in_string = "'" . implode( "','", $post_name__in ) . "'";
+					$orderby_clause = "FIELD( {$wpdb->posts}.post_name," . $post_name__in_string . ' )';
+				}
+				break;
 			default:
 				if ( array_key_exists( $orderby, $meta_clauses ) ) {
 					// $orderby corresponds to a meta_query clause.
@@ -2239,6 +2261,12 @@ class WP_Query {
 			$q['order'] = $rand ? '' : $this->parse_order( $q['order'] );
 		}
 
+		// These values of orderby should ignore the 'order' parameter.
+		$force_asc = array( 'post__in', 'post_name__in', 'post_parent__in' );
+		if ( isset( $q['orderby'] ) && in_array( $q['orderby'], $force_asc, true ) ) {
+			$q['order'] = '';
+		}
+
 		// Order by.
 		if ( empty( $q['orderby'] ) ) {
 			/*
@@ -2252,12 +2280,6 @@ class WP_Query {
 			}
 		} elseif ( 'none' == $q['orderby'] ) {
 			$orderby = '';
-		} elseif ( $q['orderby'] == 'post__in' && ! empty( $post__in ) ) {
-			$orderby = "FIELD( {$wpdb->posts}.ID, $post__in )";
-		} elseif ( $q['orderby'] == 'post_parent__in' && ! empty( $post_parent__in ) ) {
-			$orderby = "FIELD( {$wpdb->posts}.post_parent, $post_parent__in )";
-		} elseif ( $q['orderby'] == 'post_name__in' && ! empty( $post_name__in ) ) {
-			$orderby = "FIELD( {$wpdb->posts}.post_name, $post_name__in )";
 		} else {
 			$orderby_array = array();
 			if ( is_array( $q['orderby'] ) ) {
diff --git tests/phpunit/tests/post/query.php tests/phpunit/tests/post/query.php
index 3e95e73605..1112c87e04 100644
--- tests/phpunit/tests/post/query.php
+++ tests/phpunit/tests/post/query.php
@@ -174,6 +174,38 @@ class Tests_Post_Query extends WP_UnitTestCase {
 		$this->assertSame( $ordered, wp_list_pluck( $q->posts, 'ID' ) );
 	}
 
+	/**
+	 * @ticket 38034
+	 */
+	public function test_orderby_post__in_array() {
+		$posts = self::factory()->post->create_many( 4 );
+
+		$ordered = array( $posts[2], $posts[0], $posts[3] );
+
+		$q = new WP_Query( array(
+			'post_type' => 'any',
+			'post__in' => $ordered,
+			'orderby' => array( 'post__in' => 'ASC' ),
+		) );
+		$this->assertSame( $ordered, wp_list_pluck( $q->posts, 'ID' ) );
+	}
+
+	/**
+	 * @ticket 38034
+	 */
+	public function test_orderby_post__in_array_with_implied_order() {
+		$posts = self::factory()->post->create_many( 4 );
+
+		$ordered = array( $posts[2], $posts[0], $posts[3] );
+
+		$q = new WP_Query( array(
+			'post_type' => 'any',
+			'post__in' => $ordered,
+			'orderby' => 'post__in',
+		) );
+		$this->assertSame( $ordered, wp_list_pluck( $q->posts, 'ID' ) );
+	}
+
 	function test_post__in_attachment_ordering() {
 		$post_id    = self::factory()->post->create();
 		$att_ids    = array();
diff --git tests/phpunit/tests/rest-api/rest-posts-controller.php tests/phpunit/tests/rest-api/rest-posts-controller.php
index 074f5437e7..ee01eb717e 100644
--- tests/phpunit/tests/rest-api/rest-posts-controller.php
+++ tests/phpunit/tests/rest-api/rest-posts-controller.php
@@ -291,7 +291,7 @@ class WP_Test_REST_Posts_Controller extends WP_Test_REST_Post_Type_Controller_Te
 		$data     = $response->get_data();
 		$this->assertEquals( 2, count( $data ) );
 		$this->assertEquals( $id1, $data[0]['id'] );
-		$this->assertPostsOrderedBy( "FIELD( {posts}.ID, $id1,$id3 )" );
+		$this->assertPostsOrderedBy( "FIELD({posts}.ID,$id1,$id3)" );
 		// Invalid include should error
 		$request = new WP_REST_Request( 'GET', '/wp/v2/posts' );
 		$request->set_param( 'include', 'invalid' );