Synovial or ganglion cysts are the most common soft tissue tumors of the wrist and hand and can arise from joints or tendons. Intratendinous synovial cysts, in contrast, are rare and their pathogenesis is still a matter of debate. The treatment of synovial cysts of articular origin using arthroscopy is commonly used with good results. For cysts of tendon origin, that is, extra-articular, when located at the ankle, shoulder, knee, and wrist, endoscopic treatment has also been described in a procedure called tenoscopy. We describe the technique and the good results using tenoscopy for a patient treated for an intratendinous synovial cyst located at the extensor communis tendon of the third digit. It is a reproducible, safe technique with low morbidity, and the patient has had no complications or recurrence.