A 35-year-old woman visited the emergency department for a pedestrian traffic accident. Severe tenderness was noted at the posterior sacrum area, without open wound or initial neurologic deficit. Fracture of the left sacral ala extended to the S1 foramen, anterior acetabulum, and pubic ramus. Two weeks after the injury, she presented aggravating radiculopathy with the weakness of the left great toe plantar flexion. The S1 nerve root was compressed by the fracture fragments in the left S1 foramen. Decompressive S1 foraminotomy was performed. The postoperative follow-up computed tomography scan showed successful decompression of the encroachment, and the patient recovered well from the radiculopathy with motor weakness. She was able to resume her daily routine activity. We suggest that early decompressive sacral foraminotomy could be a useful additional procedure in selective sacral zone II fractures that are accompanied by radiculopathy with a motor deficit.