diff --git a/functions.php b/functions.php
index d162d1f..657127a 100644
--- a/functions.php
+++ b/functions.php
@@ -448,3 +448,11 @@ function user_registration_filter($user_id, $email) {
    return $user_id;
 }
 add_filter('email_exists', 'user_registration_filter', 10, 2);
+
+/* Security: prevent multisite signup */
+function prevent_multisite_signup()
+{
+    wp_redirect( site_url() );
+    die();
+}
+add_action( 'signup_header', 'prevent_multisite_signup' );