The victim, in his mid-20s, "was bitten by what is believed to be a shark" while he swam at Newcomb Hollow Beach in the Cape Cod area, Wellfleet Police said in a statement Saturday.
He was pulled from the water and taken to hospital by firefighters but succumbed to his injuries, the police said.
They did not release the victim's name.
"The town beaches are closed to swimming for the next 24 hours," the police said.
Several years ago a man was confirmed to have been bitten by a great white shark in the ocean near the shoreline of Truro in Cape Cod.
But the chances of being attacked by a shark are nearly one in four million, according to the International Wildlife Museum in Tucson, Arizona.
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