Maine Marine Patrol said a witness saw the woman swimming off the shore of Bailey Island when she was injured in what appeared to be a shark attack.
Two kayakers helped the person get to shore, and an ambulance provided further assistance, but she was pronounced dead at the scene, Marine Patrol said.
The name of the woman was not available on Monday.
There has only been one recorded unprovoked shark attack in Maine, New England shark expert James Sulikowski told the Portland Press Herald. Sulikowski, who researches for Arizona State University, said it's possible the shark mistook the person for food.
The state Marine Patrol is urging swimmers and boaters to use caution near Bailey Island and to avoid swimming near schooling fish or seals.