Abstract: A security device to prevent the unauthorized removal of sliding windows and doors from the frame structures in which they are slidably supported. The device is defined by a security clip having a substantially flat wall forming a mainstay and a spring-tension-jaw member integrally formed along the lower edge thereof. The security clip is arranged to be force-fitted over the depending flange of the frame structure, whereby the clip is held in a predetermined position relative to the sliding window or door. In order to prevent upward lifting of the window or door, the wall mainstay is formed having an inwardly bent, abutment lip for engagement with the window when raised from the lower frame-structure track. Ears are formed at the opposite ends of the abutment lip for engagement with the depending flange so as to reinforce and increase the strength and resistance of the abutment lip. A spacer bar is adapted to be positioned on the abutment-lip member to accommodate the various sizes of windows or doors.