A Geyser sign is a rare manifestation of acromioclavicular joint (ACJ) ganglion cysts that are seen in patients with massive rotator cuff tears and arthritis of the ACJ. We present a case report of a 70-yr-old male who was presented to our hospital with a massive AC ganglion cyst measuring 7 × 8 cm with normal shoulder function tests. Imaging studies revealed a massive rotator cuff tear, advanced cuff arthropathy, degeneration of the ACJ, and a Geyser sign (seen with magnetic resonance imaging). The cyst was initially aspirated but recurred, so open debridement distal clavicle resection and ganglion cyst excision were performed. Eight months following debridement, the patient visited the outpatient department with cyst recurrence and limited shoulder movement. A reverse shoulder arthroplasty was performed with good clinical results.