Abstract: A security device is provided for installation on double hung windows to prevent the window lock from being jimmied open from the outside and to tighten the windows sashes to the frame as well as to each other when locked. The device is in the form of a generally flat plate or shield that is installed under the window latch and is formed with a pair of fingers which extend across the gap between the upper and lower sashes on either side of the keeper. Reentrant openings are formed from the edge of the shield to allow easy installation and a central portion of reduced thickness introduces a slight tilt to the latch to provide an improved locking grip with the keeper.