The shark bite killed Julie Dimperio Holowach, 63, of New York City, the Maine Department of Marine Resources said.
The shark fatally bit Holowach off Bailey Island on Monday while she was swimming.
Name of shark attack victim released - officials urge caution for swimmers and paddlers in Casco Bay region https://t.co/DetvpACgQ5
— Maine DMR (@MaineDMR) July 28, 2020
Scientists were able to later identify the shark as a great white using a tooth fragment, state officials said.
Great whites are not common in Maine, which is the northern tip of their range, but recent summers have brought reports of sightings of the giant fish.
The Maine Marine Patrol said a witness saw Ms Holowach swimming off the shore of Bailey Island when she was bitten.
Two kayakers helped her get to shore and an ambulance provided further assistance, but she was pronounced dead at the scene, the marine patrol said.
There had previously only been one recorded unprovoked shark attack in Maine, and it was 10 years ago off Eastport, Maine Marine Resources Commissioner Patrick Keliher said. He said that bite involved a different species of shark.
He described the Monday attack as “highly unusual”, but added it is cause for vigilance among beachgoers.
“The rarity of this event does not mean it’s not going to happen again,” Mr Keliher said.
The attack happened about 30-40 feet offshore, and the rescuers were a man and woman in a rented tandem kayak, said Jeff Cooper, co-owner H2Outfitters on the island, which offers instruction, rentals and sales of kayaks.
The kayakers were on the shore when one of them saw something breach, and realised something was wrong, Mr Cooper said.
“They happened to be right there at the scene. They were courageous enough to jump in and retrieve the victim,” Mr Cooper said.
One of the kayakers grabbed Ms Holowach, and the other paddled to get her back to shore, said Cathy Piffath, the other co-owner. The kayakers asked not to be identified.
Ms Holowach and her family own property in the area and are well known in the community, which is shaken by her death, said Rob Beal, major of the Maine Marine Patrol.
The Marine Patrol has urged swimmers and boaters to use caution near Bailey Island and to avoid swimming near schooling fish or seals.
Large sharks like great whites prey on seals, which are common off Maine. Two coastal state parks, Popham and Reid, are not allowing swimmers beyond waist deep water, state officials said.
Bailey Island is part of the town of Harpswell, about 40 miles up the Maine coast from Portland.
Harpswell is a town of about 4,700 that is popular with holidaymakers and second homeowners for its coast, beaches and sweeping views of Casco Bay.