diff --git src/wp-admin/network/site-new.php src/wp-admin/network/site-new.php
index 5876037..1965e99 100644
--- src/wp-admin/network/site-new.php
+++ src/wp-admin/network/site-new.php
@@ -47,7 +47,7 @@ if ( isset($_REQUEST['action']) && 'add-site' == $_REQUEST['action'] ) {
 	// If not a subdomain install, make sure the domain isn't a reserved word
 	if ( ! is_subdomain_install() ) {
 		/** This filter is documented in wp-includes/ms-functions.php */
-		$subdirectory_reserved_names = apply_filters( 'subdirectory_reserved_names', array( 'page', 'comments', 'blog', 'files', 'feed', 'wp-admin', 'wp-content', 'wp-includes' ) );
+		$subdirectory_reserved_names = apply_filters( 'subdirectory_reserved_names', array( 'page', 'comments', 'blog', 'files', 'feed', 'wp-admin', 'wp-content', 'wp-includes', 'wp-json' ) );
 		if ( in_array( $domain, $subdirectory_reserved_names ) )
 			wp_die( sprintf( __('The following words are reserved for use by WordPress functions and cannot be used as blog names: <code>%s</code>' ), implode( '</code>, <code>', $subdirectory_reserved_names ) ) );
 	}
diff --git src/wp-includes/ms-functions.php src/wp-includes/ms-functions.php
index 464d83e..da118ef 100644
--- src/wp-includes/ms-functions.php
+++ src/wp-includes/ms-functions.php
@@ -622,7 +622,7 @@ function wpmu_validate_blog_signup( $blogname, $blog_title, $user = '' ) {
 			 *
 			 * @param array $subdirectory_reserved_names Array of reserved names.
 			 */
-			apply_filters( 'subdirectory_reserved_names', array( 'page', 'comments', 'blog', 'files', 'feed', 'wp-admin', 'wp-content', 'wp-includes' ) )
+			apply_filters( 'subdirectory_reserved_names', array( 'page', 'comments', 'blog', 'files', 'feed', 'wp-admin', 'wp-content', 'wp-includes', 'wp-json' ) )
 		);
 	}