A ganglion cyst is a fluid-filled bump associated with a joint or tendon sheath. It most often occurs at the back of the wrist, followed by the front of the wrist. Onset is often over several months, typically with no further symptoms. Occasionally, pain or numbness may occur. Complications may include carpal tunnel syndrome.

The cause is unknown. The underlying mechanism is believed to involve an outpouching of the synovial membrane. Risk factors include gymnastics activity. Diagnosis is typically based on examination with light shining through the lesion being supportive. Medical imaging may be done to rule out other potential causes.

Treatment options include watchful waiting, splinting the affected joint, needle aspiration, or surgery. About half the time, they resolve on their own. About three per 10,000 people newly develop ganglion of the wrist or hand a year. They most commonly occur in young and middle-aged females.