Synovitis is the medical term for inflammation of the synovial membrane. This membrane lines joints that possess cavities, known as synovial joints. The condition is usually painful, particularly when the joint is moved. The joint usually swells due to synovial fluid collection.

Synovitis may occur in association with arthritis as well as lupus, gout, and other conditions. Synovitis is more commonly found in rheumatoid arthritis than in other forms of arthritis, and can thus serve as a distinguishing factor, although it is also present in many joints affected with osteoarthritis. In rheumatoid arthritis, the fibroblast-like synoviocytes, highly specialized mesenchymal cells found in the synovial membrane, play an active and prominent role in the synovitis. Long term occurrence of synovitis can result in degeneration of the joint.