Julie Holowach, 63, died Monday after a shark bit her while she swam near Bailey Island.
Patrick Keliher of the Maine Department of Marine Resources said a recovered tooth matched those belonging to a great white shark.
Holowach, of New York City, was about 60 feet from shore when she was fatally bitten. Nearby kayakers helped the woman to shore, where emergency medical officials pronounced her dead around 3:20 p.m.
Keliher described the attack as rare.
“It is the only confirmed fatality in Maine waters from a shark attack,” he said.
He urged swimmers to take caution and avoid swimming near schools of fish or seals.
“Seals feed on fish and sharks feed on seals,” he said.
“We urge swimmers and others recreating in or on the water in the Casco bay region, and in particular near Bailey Island to be aware of their surroundings and to avoid schools of fish or seals, which attract sharks.”