A security device for installation in an upper sash of a double-hung window, in which the window has a window frame having two side surfaces, a top surface and a bottom surface, a lower sash mounted in a lower half of the window frame and an upper sash mounted in an upper half of the window frame. The upper and lower sashes are mounted offset from each other so that they slide up and down the window frame independently of each other. The security device is mounted to the upper sash via a bracket, and comprises a post having a top end that abuts the top surface of the window frame and a bottom end abutting a top surface of the lower sash to prevent the lower sash from sliding within the window frame. There is a cross-bar running through the post perpendicular to a longitudinal axis of the post and extending in opposite directions from the post. The cross-bar is located immediately beneath the bracket to act as a stop against downward sliding of the upper sash.