Traumatic myositis ossificans in the masseter muscles has rarely been reported. Traumatic myositis ossificans usually occurs in young males as a result of direct trauma. Wedescribe a 31-year-old man with severe trismus caused by traumatic myositis ossificans. The patient was under confinement for about 3 months and abused his face frequently. As a result, he was unable to open his mouth. The maximum interincisal opening was 5 mm. A hard board-like mass was palpated in bilateral cheeks. Physical examination revealed that he was healthy, except for severe trismus. On laboratory examination, the alkaline phosphatase level was twice the upper limit of normal. Diagnosis was based on clinical findings plus radiography, CT, and 3 D-CT. Partial removal of the masseter was performed, achieving an interincisal opening of 37mm. Surgical specimen showed heterotopic ossification. Postoperatively, he could not open his mouth, because his lip and cheek had cicatrized. Despite active physiotherapy, trismus reappeared. Subsequently, mouth opening training continued. After 10 months, the patient could open his mouth, with a maximum interincisal opening of 35mm.