"I went out for a quiet dip and had swum out a few meters when I noticed something underneath the water. Next thing I felt a pain in my thigh and up came the head of the beaver," Sofie Adde, 40, told The Local.
"It gave me quite a surprise," she said in the report Monday.
"I wasn't sure what was going on as the beaver [dived] back down and I got scared and swam away immediately."
Adde said she went to a hospital and received several stitches and a tetanus shot.
Magnus Enquist, a professor in animal behavior at Stockholm University, said it is not unusual for a beaver to attack if someone gets too close to it.
"Beavers are just like other wild animals as they get defensive but it is rare to even get that close to them," Enquist said.
"Next time I will go somewhere else when I fancy a swim," Adde said. "I will be a bit more careful as I don't think I would even let my dog swim in the same lake as the beavers."