This case report describes a horizontal intra-articular dislocation of the right patella in a 57-year-old male. This is an extremely rare injury. Our case is unusual in that previous authors have described this injury as exclusive to adolescents. A mechanism has been suggested for the causation of this injury. Our case does not seem to conform to the mechanism described, in that the patient's knee was locked in extension, not flexion, at the time of injury.