Abstract: The present invention provides an early warning sound alarm system for signalling unauthorized entry or attempted entry through a pet door when the pet door is covered and not in use. The alarm system in accordance with the invention, comprises in combination: a pet door including a frame defining a passageway therethrough, and a flap swingably mounted on the frame adjacent the upper edge of the passageway for swinging movement in either direction therethrough; a covering member removably secured to one side of the frame adjacent the flap for preventing entry through the passageway; and an electrically powered sound alarm, including a sound signal box, and a switch such as a leaf spring biased limit switch disposed between the flap and the surface of covering member adjacent the flap. The switch is suitably positioned to maintain the sound signal box in a deactivated state when the covering member is secured on the frame to prevent entry through the passageway.