An 82-year-old man presented with neck pain, right upper limb radiculopathy and right shoulder pain. Physical examination revealed a soft lump over the right shoulder joint, as well as reduced range of shoulder movements. On magnetic resonance imaging, the soft lump was shown to be a cystic mass over the acromioclavicular joint and was related to a full-thickness supraspinatus tendon tear. This is the classic geyser sign. The pathophysiology and clinical features of the geyser sign, and its imaging features with various imaging modalities, are discussed.