Pigmented villonodular synovitis (PNVS) is a rare and disabling disease that is thought to occur mostly in the knee joint. Here, the authors first present a unique case of PNVS occurring at the sacroiliac joint. The patient complained of sacroiliac joint pain with mild swelling. CT and MRI showed that the tumor was ~63 by 91 by 107 mm in size, and was considered to be a fibrous borderline or low-grade malignancy. Intraoperative macroscopic features of the synovitis during operation suggested PNVS, which was confirmed by histopathological examination. The clinical symptoms and signs of the disease, in this case, are not obvious, and radiological investigations, including MRI, suggest high aggressiveness. The author believes that it may be more likely to relapse and metastasis and recommends complete removal of the synovial membrane and regular follow-up, while preoperative or postoperative radiotherapy and molecular targeted therapy are not recommended for the time being.