Myositis ossificans is a benign localized self-limiting ossifying soft tissue mass with unknown pathogenesis. It may be confused with a malignant tumor of bone or soft tissues especially in the early stages of the disease. In this report we present a case of myositis ossificans affecting unilateral psoas muscle which to our knowledge is a very uncommon location. There was no history of trauma. We describe the imaging findings and clues to early diagnosis of myositis ossificans.