We treated a 37-year-old male with traumatic hematoma in the suprapatellar bursa that had developed in the form of persistent swelling on suprapatellar area of left knee after blunt trauma. Though there were no obvious abnormal findings on plain roentgenographs, an isolated suprapatellar cystic lesion with fluid-fluid level on T2-weighted sagittal image of MRI was noted. We found the suprapatellar plica with complete septum and no synovitis in the knee joint proper by arthroscopy. We incised the plica and found leakage of blood-stained fluid from the suprapatellar bursa. There were no findings of pigmented villonodular synovitis or other tumorous lesions. At 6 months after surgery, the patient felt symptom-free and there was no recurrence.