A vehicle-motion based vehicle theft detection device uses preexisting vehicle motion sensors in combination with an engine immobilizer system to detect and deter potential vehicle theft. Depending on whether or not a valid ignition key is in use, the engine immobilizer system sends signals to an electronic module to enter either a "disarmed" or "armed" state. When a vehicle sensor relays a signal to an electronic module, the electronic module determines, based on this signal, if the vehicle is in motion. If a valid key is in use, the electronic module is in a disarmed state and takes no further action, as the motor vehicle is authorized for motion. If, however, a valid key is not in use, the electronic module is in an armed state, and activates the vehicle alarm system which, in turn, may activate an audible or visual alarm, activate a remote notification device, generate a tracking signal, and/or store evidence of unauthorized vehicle motion in memory.