A window sash restraint device for selectively restraining a window sash in one of an open position and a closed position relative to a window frame. The window sash restraint device includes a body adapted to be fixed directly to the frame or sash and a restraining member which in an active condition, restrains the sash in either the open position or the closed position relative to the frame and when inactive allows movement of the sash relative to the opened or closed positions. The device further comprises a manually releasable restraining mechanism for holding the restraining member in the active condition. The device is adapted to restrain the sash in the opened position, with an opening small enough to prevent a child from falling out of the window yet secure enough to prevent the child from further opening and falling from the window.