Temporomandibular ankylosis is defined as a permanent constriction of the temporomandibular joint. It can be an articular or juxta-articular origin, and includes various pathologies. We report here an unusual cause of temporo mandibular ankylosis which is an osteochondroma diagnosed by an anatomopathological study of the bone block. A 16-year-old patient who consults for a difficulty in opening the mouth. The clinical examination was without particularities. The cranian scan revealed the presence of a bony block at the right temporo mandibular joint with almost complete disappearance of the joint space.It was the histological examination of the osseous block that led to the diagnosis of osteochondroma.