South Beach on Martha's Vineyard was closed for a short time, and swimmers were kept out of the water at State Beach in Edgartown, the Massachusetts Department of Conservation and Recreation said.
A plane was dispatched to try to confirm the sighting, but no shark was spotted, said Lisa Capone, a spokeswoman for the state Executive Office of Energy and Environmental Affairs.
In 1974, Steven Spielberg chose Martha's Vineyard for filming the movie version of "Jaws," depicting a series of deadly great white attacks in the fictional community of Amity.
Shark attacks are extremely rare in waters off New England, but great whites have been known to occasionally prowl in the region.
