Index: src/wp-admin/includes/ms.php
===================================================================
--- src/wp-admin/includes/ms.php	(revision 33141)
+++ src/wp-admin/includes/ms.php	(working copy)
@@ -190,6 +190,13 @@
 
 	if ( !$user->exists() )
 		return false;
+
+	// Global super-administrators are protected, and cannot be deleted.
+	$_super_admins = get_super_admins();
+	if ( in_array( $user->user_login, $_super_admins, true ) ) {
+		return false;
+	}
+
 	/**
 	 * Fires before a user is deleted from the network.
 	 *
Index: tests/phpunit/tests/user/multisite.php
===================================================================
--- tests/phpunit/tests/user/multisite.php	(revision 33141)
+++ tests/phpunit/tests/user/multisite.php	(working copy)
@@ -229,6 +229,28 @@
 		$this->assertQueryTrue( 'is_author', 'is_archive' );
 	}
 
+	function test_super_admin_cannot_be_deleted() {
+		$user_id = $this->factory->user->create();
+		grant_super_admin( $user_id );
+		$this->assertFalse( wpmu_delete_user( $user_id ) );
+	}
+
+	function test_revoked_super_admin_can_be_deleted() {
+		$user_id = $this->factory->user->create();
+		grant_super_admin( $user_id );
+		revoke_super_admin( $user_id );
+		$this->assertTrue( wpmu_delete_user( $user_id ) );
+	}
+
+	function test_revoked_super_admin_is_deleted() {
+		$user_id = $this->factory->user->create();
+		grant_super_admin( $user_id );
+		revoke_super_admin( $user_id );
+		wpmu_delete_user( $user_id );
+		$user = new WP_User( $user_id );
+		$this->assertFalse( $user->exists(), 'WP_User->exists' );
+	}
+
 	/**
 	 * @ticket 27205
 	 */