The sash lock has a rotatable cam which engages the keeper. The sash lock has a slide plate mounted below the cam and the slide plate moves between a retracted position in which its leading edge is flush with the sash to which the lock is mounted. In an extended position in which the leading edge of the plate extends far enough to block the narrow gap between the sashes. The slide plate is mounted for movement perpendicular to the edge of the sash and parallel to the plane of the top surface of the sash. The slide plate includes an arcuate recess in one side of the plate which is engaged by a pin extending from the rotatable cam. The recess in the slide plate is contoured so that during initial portion of the movement of the cam from its open position toward its locked position, the slide plate does not move. After the cam has moved far enough to engage the keeper, the slide plate moves outward toward the sash to which the keeper is mounted so as to block the gap between the two sashes in the region of the cam. It reduces the chances of a sash lock being forced open.