diff --git a/src/wp-includes/media.php b/src/wp-includes/media.php
index deb18a40b6..3411b84c80 100644
--- a/src/wp-includes/media.php
+++ b/src/wp-includes/media.php
@@ -3679,19 +3679,33 @@ function get_post_galleries( $post, $html = true ) {
 			if ( 'gallery' === $shortcode[2] ) {
 				$srcs = array();
 
+				$shortcode_attrs = shortcode_parse_atts( $shortcode[3] ); 
+				if ( ! is_array( $shortcode_attrs ) ) {
+					$shortcode_attrs = array();
+				}
+
+				// Specify the post id of the gallery we're viewing if the shortcode doesn't reference another post already.
+				if ( ! isset( $shortcode_attrs['id'] ) ) {
+					$shortcode[3] .= ' id="' . intval( $post->ID ) . '"';
+				}
+
 				$gallery = do_shortcode_tag( $shortcode );
 				if ( $html ) {
 					$galleries[] = $gallery;
 				} else {
 					preg_match_all( '#src=([\'"])(.+?)\1#is', $gallery, $src, PREG_SET_ORDER );
 					if ( ! empty( $src ) ) {
-						foreach ( $src as $s )
+						foreach ( $src as $s ) {
 							$srcs[] = $s[2];
+						}
 					}
 
-					$data = shortcode_parse_atts( $shortcode[3] );
-					$data['src'] = array_values( array_unique( $srcs ) );
-					$galleries[] = $data;
+					$galleries[] = array_merge(
+						$shortcode_attrs,
+						array(
+							'src' => array_values( array_unique( $srcs ) )
+						)
+					);
 				}
 			}
 		}
diff --git a/tests/phpunit/tests/media.php b/tests/phpunit/tests/media.php
index d8c28c2191..af8c2f1428 100644
--- a/tests/phpunit/tests/media.php
+++ b/tests/phpunit/tests/media.php
@@ -390,6 +390,89 @@ BLOB;
 		$this->assertEquals( $srcs, array( $ids1_srcs, $ids2_srcs ) );
 	}
 
+	/**
+	 * @ticket 39304
+	 */
+	function test_post_galleries_images_without_global_post() {
+		// Set up an unattached image.
+		$this->factory->attachment->create_object( array(
+			'file' => 'test.jpg',
+			'post_parent' => 0,
+			'post_mime_type' => 'image/jpeg',
+			'post_type' => 'attachment'
+		) );
+
+		$post_id = $this->factory->post->create( array(
+			'post_content' => '[gallery]',
+		) );
+
+		$galleries = get_post_galleries( $post_id, false );
+
+		$this->assertEmpty( $galleries[0]['src'] );
+	}
+
+	/**
+	 * @ticket 39304
+	 */
+	function test_post_galleries_ignores_global_post() {
+		$global_post_id = $this->factory->post->create( array(
+			'post_content' => 'Global Post',
+		) );
+		$post_id = $this->factory->post->create( array(
+			'post_content' => '[gallery]',
+		) );
+		$this->factory->attachment->create_object( array(
+			'file' => 'test.jpg',
+			'post_parent' => $post_id,
+			'post_mime_type' => 'image/jpeg',
+			'post_type' => 'attachment'
+		) );
+		$expected_srcs = array(
+			'http://' . WP_TESTS_DOMAIN . '/wp-content/uploads/test.jpg'
+		);
+
+		// Set the global $post context to the other post.
+		$GLOBALS['post'] = get_post( $global_post_id );
+
+		$galleries = get_post_galleries( $post_id, false );
+
+		$this->assertNotEmpty( $galleries[0]['src'] );
+		$this->assertSame( $galleries[0]['src'], $expected_srcs );
+	}
+
+	/**
+	 * @ticket 39304
+	 */
+	function test_post_galleries_respects_id_attrs() {
+		$post_id = $this->factory->post->create( array(
+			'post_content' => 'No gallery defined',
+		) );
+		$post_id_two = $this->factory->post->create( array(
+			'post_content' => "[gallery id='$post_id']",
+		) );
+		$this->factory->attachment->create_object( array(
+			'file' => 'test.jpg',
+			'post_parent' => $post_id,
+			'post_mime_type' => 'image/jpeg',
+			'post_type' => 'attachment'
+		) );
+		$expected_srcs = array(
+			'http://' . WP_TESTS_DOMAIN . '/wp-content/uploads/test.jpg'
+		);
+
+		$galleries = get_post_galleries( $post_id_two, false );
+
+		// Set the global $post context
+		$GLOBALS['post'] = get_post( $post_id_two );
+		$galleries_with_global_context = get_post_galleries( $post_id_two, false );
+
+		// Check that the global post state doesn't affect the results
+		$this->assertSame( $galleries, $galleries_with_global_context );
+
+		$this->assertNotEmpty( $galleries[0]['src'] );
+		$this->assertSame( $galleries[0]['src'], $expected_srcs );
+	}
+
 	/**
 	 * @ticket 22960
 	 */