A compact wireless security sensor having a magnetically operated plunger switch. The compact nature of the sensor of the present invention makes it ideal for being substantially concealing into a door frame or window as part of a wireless security system. The sensor unit includes a housing having an inner end, an outer end, and a magnet positioned within a moveable plunger, and an antenna, preferably a flexible wire antenna. The housing further contains a sensor switch, a microprocessor with a PCB, a wireless transmitter, such as an RF transmitter, and a power source, such as a small coin cell battery, for emitting signals to a master station or controller when the plunger switch is depressed and activates the internal sensor switch.