Os trigonum fractures are seen rarely. Our patient was a 16-year-old male patient who applied to our emergency service with right ankle pain due to acute hyperplantar flexion while playing football. There was a swelling on the right ankle and he had difficulty in walking. Os trigonum fracture was detected at lateral X-ray of the right ankle. No additional injury was detected with computed tomography and magnetic resonance imaging (MRI). Patient was treated conservatively. A short leg cast was applied. Cast was removed after six weeks and range of motion exercises were started. Union of fracture was seen on X-ray at six weeks from injury. Conservative treatment is thought to be a good option in these fractures. Computed tomography and MRI are important in the differential diagnosis and evaluation of additional injuries in this type of fracture.