Leah Prudhomme, a 33-year-old triathlete from Anoka, was swimming Saturday night in Island Lake, about 17 miles north of Duluth, when she was attacked, Minnesota Public Radio reported.
Prudhomme said she was nearing the end of her swim when felt something bite her ankle to the bone. She stopped moving to see what it was.
"It had a gray head, little beaded eyes, and was very agile in the water. And it would dive down, come back over to where I am -- I can't see where it's going -- to bite me because the water's so dark, and it would just like latch onto my leg and latch onto my thigh," Prudhomme said.
She was bitten 25 times by the otter before her father rescued her in a boat, Prudhomme said.
"My wetsuit tells the tale best because there's just claw marks and chunks missing and lots of bites all over the wetsuit," she said. "It's pretty much destroyed now."
After the attack, Prudhomme went to a local hospital for rabies, tetanus and antibiotic shots.
Otters attacking humans is typically very rare, the Minneapolis Star Tribune reported.
Experts and doctors said the aggressive otter that attacked Prudhomme was likely protecting young pups.
"I've never seen or heard of it before," said Mike Scott, a conservation officer with the Minnesota Department of Natural Resources in Duluth. "We've got otters everywhere ... lakes, streams. Most times, [swimmers] wouldn't even know it. Otters usually stay away."