A 62-year-old male presented with a rare case of ligamentum flavum hematoma manifesting as low back pain and gait difficulty beginning 1 month before consulting our institute. He had no history of lumbar spine surgery or lumbar puncture. However, he might have suffered forgotten back injury while practicing martial arts. Magnetic resonance imaging showed a heterogeneous intensity mass lesion with a cystic component at the L3-4 levels. The lesion was totally removed through a hemilaminectomy. Intraoperative and histological findings confirmed the diagnosis of old hematoma with granulomatous change in the ligamentum flavum. Postoperatively, his low back pain and gait difficulty resolved within a few days.