Abstract: An anti-theft device for a lower unit of an outboard engine including a collar extending around the outboard engine, pivoting apparatus operatively connected to the collar, and locking apparatus operatively connected to the collar and selectively maintaining the collar around the outboard engine. The collar includes a pair of legs pivotally attached to each other by the pivoting apparatus and longitudinally curved to conform to, extend along, and capture, the cavitation plate of the outboard engine. A pair of plates extend inwardly from the pair of legs and are shaped and positioned so as to cover an opposing pair of mounting bolts and any space therebetween so as to deny access thereto when the anti-theft device is in place. The terminal edges of the plate are tongue and grooved, respectively, so as to form a tongue and groove joint when the anti-theft device is in place.