Orleans Harbormaster Dawson Farber told the Cape Cod Times that a senior lifeguard spotted a dorsal fin 150 yards off Nauset beach Sunday morning.
Swimmers were asked to get out of the water and the beach remained closed for about an hour. Nearby towns were also notified of the sighting.
State shark expert Greg Skomal has not yet confirmed that the animal, thought to be between 12 and 13 feet, was in fact a great white shark.
Friday state biologists confirmed a great white shark tagged in 2011 was spotted Tuesday off the coast of Cape Cod.