A portable door alarm including a front component connected to a rear component by a connecting component, the connecting component configured to be positioned at an edge of a door such as between a floor and a bottom edge of a door to which the alarm is associated so that the front component is positioned on a first side of the door and the rear component is positioned on an opposite side of the door, the rear component including an optical motion sensor configured to detect movement of the door. In one aspect the alarm is configured to fit upon a bottom edge of the door and to slide with the door as the door is opened or closed. In other aspects the front component includes an indicator and the rear component may include a variety of electrical components for various operational features. Various methods of operation are also provided.