diff --git a/wp-includes/blocks.php b/wp-includes/blocks.php
index ed0c4d2dbf..f65ddb6c2e 100644
--- a/wp-includes/blocks.php
+++ b/wp-includes/blocks.php
@@ -262,3 +262,17 @@ function _recurse_do_blocks( $blocks, $all_blocks ) {
 
 	return $rendered_content;
 }
+
+/**
+ * Returns the current version of the block format that the content string is using.
+ *
+ * If the string doesn't contain blocks, it returns 0.
+ *
+ * @since 5.0.0
+ *
+ * @param string $content Content to test.
+ * @return int The block format version.
+ */
+function block_version( $content ) {
+	return has_blocks( $content ) ? 1 : 0;
+}
diff --git a/wp-includes/rest-api/endpoints/class-wp-rest-posts-controller.php b/wp-includes/rest-api/endpoints/class-wp-rest-posts-controller.php
index 5002484ed8..93d2f0806f 100644
--- a/wp-includes/rest-api/endpoints/class-wp-rest-posts-controller.php
+++ b/wp-includes/rest-api/endpoints/class-wp-rest-posts-controller.php
@@ -1521,10 +1521,11 @@ class WP_REST_Posts_Controller extends WP_REST_Controller {
 
 		if ( in_array( 'content', $fields, true ) ) {
 			$data['content'] = array(
-				'raw'       => $post->post_content,
+				'raw'           => $post->post_content,
 				/** This filter is documented in wp-includes/post-template.php */
-				'rendered'  => post_password_required( $post ) ? '' : apply_filters( 'the_content', $post->post_content ),
-				'protected' => (bool) $post->post_password,
+				'rendered'      => post_password_required( $post ) ? '' : apply_filters( 'the_content', $post->post_content ),
+				'protected'     => (bool) $post->post_password,
+				'block_version' => block_version( $post->post_content ),
 			);
 		}
 
@@ -2062,18 +2063,24 @@ class WP_REST_Posts_Controller extends WP_REST_Controller {
 							'validate_callback' => null, // Note: validation implemented in self::prepare_item_for_database()
 						),
 						'properties'  => array(
-							'raw'       => array(
+							'raw'           => array(
 								'description' => __( 'Content for the object, as it exists in the database.' ),
 								'type'        => 'string',
 								'context'     => array( 'edit' ),
 							),
-							'rendered'  => array(
+							'rendered'      => array(
 								'description' => __( 'HTML content for the object, transformed for display.' ),
 								'type'        => 'string',
 								'context'     => array( 'view', 'edit' ),
 								'readonly'    => true,
 							),
-							'protected' => array(
+							'block_version' => array(
+								'description' => __( 'Version of the content block format used by the object.' ),
+								'type'        => 'integer',
+								'context'     => array( 'edit' ),
+								'readonly'    => true,
+							),
+							'protected'     => array(
 								'description' => __( 'Whether the content is protected with a password.' ),
 								'type'        => 'boolean',
 								'context'     => array( 'view', 'edit', 'embed' ),
diff --git a/wp-includes/version.php b/wp-includes/version.php
index 089e75457d..6b30e1dbe0 100644
--- a/wp-includes/version.php
+++ b/wp-includes/version.php
@@ -13,7 +13,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '5.1-alpha-44126';
+$wp_version = '5.1-alpha-44127';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.