diff --git a/wp-includes/ms-settings.php b/wp-includes/ms-settings.php
index 3e10ba56fc..892548f5de 100644
--- a/wp-includes/ms-settings.php
+++ b/wp-includes/ms-settings.php
@@ -172,10 +172,10 @@ if ( !isset( $current_site ) || !isset( $current_blog ) ) {
 	if ( ! isset( $current_site->blog_id ) ) {
 		if ( $current_blog->domain === $current_site->domain && $current_blog->path === $current_site->path ) {
 			$current_site->blog_id = $current_blog->blog_id;
-		} else {
-			// @todo we should be able to cache the blog ID of a network's main site easily.
+		} elseif ( ! $current_site->blog_id = wp_cache_get( 'network:' . $current_site->id . ':main_site', 'site-options' ) ) {
 			$current_site->blog_id = $wpdb->get_var( $wpdb->prepare( "SELECT blog_id FROM $wpdb->blogs WHERE domain = %s AND path = %s",
 				$current_site->domain, $current_site->path ) );
+			wp_cache_add( 'network:' . $current_site->id . ':main_site', $current_site->blog_id, 'site-options' );
 		}
 	}