Swimming in the 10 km freestyle, a new event at the forthcoming Olympic Games, Berglund was attacked by the jellyfish towards the end of the marathon race, Dagens Nyheter reports.
"It was 400 metres from the finish. The underside came right at my face. I flailed about with my arms and didn't think I would make it to the finish.
"It hurt in the same way as when you're stung by nettles, only a hundred times worse. I wanted to tear off my skin," Berglund told Dagens Nyheter.
With an ice pack pushed to her face and crying from the pain, Berglund was taken for treatment.
Incredibly, she managed to finish the race under the watchful eyes of Swedish team coach Thomas Jansson. Her tenth place finish, just 16 seconds after six-time world champion, 18-year-old Larisa Iltjenko, puts her in serious contention for an Olympic place.
"She beat reigning European champion Rita Kovacs. And in second place was Cassandra Patten, who took gold at the 400 metre freestyle event in the World Championships two years ago.
"That shows the quality and still she was up there. This and her eighth place in the 5 km event means that she should be very well placed for the Olympics," said Thomas Jansson.