Abstract: A pet door includes an elongated, rectangular panel having a first side edge hingedly attached to the free, pivotal side edge of an exterior door. The panel is formed of an upper section that is foldable onto a lower section to allow the device to be compactly transported or stored. The lower section includes an opening having a movable hatch positioned therein that allows a pet to freely pass through the panel. The upper section of the panel includes a triangular flap hingedly attached to a top edge that is securable to the upper edge of the exterior door. Accordingly, the exterior door is partially opened and the panel is pivoted against the door frame to block the space formed therebetween. The flap is secured to the top edge of the door to overlay the triangular space formed between the exterior door and frame header. Therefore, the panel forms a secure barrier between the open door and the exterior while allowing a pet to freely enter and exit the building.