The invention discloses a window stopper. The window stopper comprises a safety ring and two positioning locks jointed on the safety ring; and every positioning lock comprises a base, and a sliding block, a first fixing block, a second fixing block and an adjusting rod which are connected to the base; the first fixing block is perpendicularly fixedly connected to one end of the bottom surface of the base, the base is provided with a first through hole and a second through hole, the first through hole penetrates through the base bilaterally, the sliding block is arranged in the first through hole, one side, close to the first fixing block, of the sliding block is perpendicularly fixedly connected with the second fixing block, the first fixing block is arranged in parallel with the second fixing block and is used for clamping and fixing a window, the second through hole is arranged in the front surface of the base, the second through hole communicates the front surface of the base with the first through hole, and the adjusting rod is arranged in the second through hole and is used for adjusting the position of the sliding block in the first through hole. The stopper has a simple structure, is convenient to mount and dismount, and is suitable for casement windows and push-pull windows.