Swimmers were ordered out of the water on Martha's Vineyard's South Beach after a reported shark sighting on Wednesday afternoon.
Lifeguards and swimmers said they spotted a fin off shore around 2 p.m.
Two shark sightings were reported on Wednesday within hours of each other, but it isn't clear if it might have been the same shark.
Swimmers were allowed back into the water around 3 p.m. The U.S. Coast Guard is continuing to monitor the situation.
The Vineyard Gazette is reporting that the shark that was spotted was an 8-foot-long hammerhead.
Tony LaCasse of the New England Aquarium said hammerhead sharks are spotted off the Massachusetts coast every summer in small numbers.