Pigmented villonodular synovitis is an extremely rare disease in skeletally immature patients. Erosive destruction of the involved joint leads to early arthritis, and its high recurrence rate makes treatment challenging. Multiple surgical approaches exist, but it is unclear as to which among them achieves the lowest possible recurrence rate and morbidity. We report the case of a 5-year-old girl with left knee pain and swelling who was diagnosed with diffuse pigmented villonodular synovitis of the left knee based on MRI findings. Combined open and arthroscopic surgery was performed to completely remove the tumor. Postoperative histopathological examination confirmed the diagnosis of diffuse pigmented villonodular synovitis. The postoperative course was uneventful, with a gradual improvement in symptoms. There were no signs of recurrence on postoperative MRI performed at the 8-month follow-up, with neither knee pain nor limitation of range of motion. The favorable outcome in this case suggests that combined open and arthroscopic surgery may be an effective method for treating pigmented villonodular synovitis in skeletally immature patients.