She was wading off the shore of Bailey Island around 3:30 p.m. when a witness saw her being attacked by what looked like a shark, Maine Marine Patrol said.
A pair of kayakers helped her get to shore, and an ambulance provided further assistance, but she was pronounced dead at the scene, the authorities said.
The swimmer’s name wasn’t released, pending notification of her family.
The incident would be only the second recorded unprovoked shark attack in the state, the Portland Press Herald reported.
“Shark interactions with humans are very rare in Maine,” New England shark expert James Sulikowski told the newspaper.
“My guess is that the person was mistaken as a food item.”
Sulikowski, who researches for Arizona State University, said he’s fairly certain the big fish was a great white shark, a species of fast swimmers who can reach lengths of more than 15-feet.
More of the large predators have been seen off the coast of Maine in recent years, he said.
The same animal may have also attacked a seal in Phippsburg on Sunday, before heading south, Sulikowski told the paper.
The Marine Patrol urged swimmers and boaters to be cautious, and Sulikowski also warned people not to wade near seals.
“We can easily be mistaken for a seal… as a shark’s dinner,” he said.