Synovitis is a condition that affects the smooth lining of a joint or tendon, called the synovial membrane. Different joints or tendons can be affected, depending on the cause. Causes include some types of arthritis, such as rheumatoid arthritis or psoriatic arthritis, and other conditions.

Symptoms might include pain and swelling. The joint might feel warm. The tissues of the joint might weaken, causing the joint to be unstable.

Treatments depend on the cause and which joints or tendons are involved. Treatments might include medicines, surgery and physical therapy.