Two other women were also injured by the furry water creature, police have said.
The attack happened near the town of Cardwell on a remote stretch of the Jefferson River, a tributary of the Missouri River which is popular for fishing and recreational use.
At least one otter swam up to the group of women, who were floating on rubber rings, at around 8.15pm on Wednesday and attacked them, according to Morgan Jacobsen from Montana Fish Wildlife and Parks.
The trio were able to get to shore, where one of them called the emergency services, he said.
One of the woman's wounds, on her face and arms, were so severe she had to be taken to hospital by helicopter, Jefferson County Undersheriff James Everett said.
Her condition is currently unknown. The others had injuries to their arms.
"It's just not something you run into very often," Mr Everett said.
"Bears do it, moose too and occasionally a deer, but otters? That's not normal."
Mr Jacobsen said one of the women saw two otters beforehand, but it was unclear how many were involved in the attack.
Northern river otters are members of the weasel family and can reach up to 9 kg - as heavy as a small dog - and up to 1.2 metres long. They primarily eat fish.
Otters can can use their teeth and claws to bite and scratch, Jacobsen said.
"If folks are attacked by an otter, our recommended response is to fight back, get away and get out of the water," he said.
Warning signs were put up at access points along the Jefferson River in the area of the attack.
No otters have been seen there since Wednesday.
There will be no efforts to catch or remove any of the animals because it's believed to have been a defensive attack, he said.
Last month in California, a sea otter gained widespread attention for aggressively wrestling surfboards from surfers off the coast of Santa Cruz.
Video and photographs shared on social media show the animal getting on top of different surfboards - on at least one occasion biting and tearing chunks off a board - and aggressively approaching surfers.
The five-year-old female otter, which officials named sea otter 841, poses a public safety risk, according to the US Fish and Wildlife Service.
A team of wildlife experts with the California Department of Fish and Wildlife and the nearby Monterey Bay Aquarium are now working to capture and put the animal in a new home, the service said.
Aren't they just painting a picture that states its NOT safe to go outdoors.