Choroidal nodular synovitis, known as hyperpigmented choroidal synovitis, is a synovitis characterized by synovial hyperplasia, massive joint effusion, and bone erosion, which was described in detail by Jaffe in 1941 and whose etiology is unclear. Current research suggests that it is a synovial inflammatory reaction, and although classified as a benign tumor, the lesion is a “malignant” proliferation. It occurs in the large joints of the extremities, mostly in the knee, but also in the hip, ankle and elbow joints. The disease can develop in a single joint or in multiple joints, and is classified as limited or diffuse depending on the location and recurrence of the lesion. The main symptoms are progressive swelling of the affected joint with localized pain, pressure and limitation of movement. The disease can be initially diagnosed by MRI, but ultimately needs to be confirmed by histopathological examination. MRI is sensitive and clearly shows the degree of synovial thickening and fluid accumulation, the villous or nodular elevation of the synovium, and the soft tissue masses around the joint space. Because of the deposition of iron-containing heme in the diseased synovial tissue, there is a low signal on both T1-weighted and T2-weighted images. This is a specific sign for the diagnosis of hyperpigmented villous nodular synovitis by MRI. Treatment is primarily synovectomy with scraping of the already eroded cartilage as well. In diffuse synovitis, the key to treatment is thorough debridement of the diseased synovial tissue, but it is often difficult to clean it with conventional surgery, and full debridement of the joint can be achieved arthroscopically, requiring good arthroscopic skills on the part of the operator. Some studies suggest that postoperative radiation therapy can prevent recurrence, but the key to treatment is complete debridement of the diseased synovium.