Abstract: The present invention is a security window insert assembly retrofit to existing insulated windows (double or other multi-pane windows). The assembly generally comprises a tubular steel gridwork anchored in a substantially rectangular spacer, and opposing panes of glass adhered to the sides of the spacer such that the gridwork occupies and is suspended in the airspace between the panes of glass. The tubular gridwork comprises a gridwork of or more horizontal and one or more vertical struts leading to distal ends press-fit into the spacer. The result is a double pane window unit that may be installed within a window frame in a conventional manner. However, the security grating erects an impenetrable shield between the interior surfaces of the panes, and since it remains anchored in the spacers it cannot be removed.