ÖTILLÖ is an ultra-endurance race held annually on the first Monday in September in the Stockholm archipelago, Sweden. 160 women's, men's and mixed teams complete a 70 km racecourse, consisting of a 60 km trail running and 10 km of open water swimming across 25 islands to crown the ÖTILLÖ Swimrun World Champions.

The essence of swimrun is racing in teams of two on a marked course in wild nature. The athletes alternate between trail running and open water swimming. To lose no time between these two disciplines, they run in their wetsuit and swim in their shoes.

In 2002 Anders Malm, one of the owners of Utö Värdshus^[1] (the finish line hotel of ÖTILLÖ, The Swimrun World Championship), his friend Janne Lindberg and some of his staff (the Andersson brothers) had a late night in the bar. Playing with a napkin with a map of the Stockholm archipelago, they challenged each other – “Last team of two to Sandhamn pays for hotel, dinner and drinks”. Two teams of two started the next morning with the only rule being that they had to pass the three different restaurants on the islands between the start and the finish. The last team at the restaurant had to drink and pay what the team ahead of them had ordered for them. It took them more than 24 hours and they were too tired to party on arrival. We call them the Original 4.

This was the first recorded Swimrun in history. In 2006, the Swedish adventure racer duo Michael Lemmel and Mats Skott turned this bet into a commercial race called ÖTILLÖ, which is Swedish and means island to island. The first couple of years only 11 teams started and only two managed to finish within the time limits. From 2007 onward, the ÖTILLÖ race underwent several changes. Participants experimented with equipment like surfboards, snorkels, fins on shoes, and creatively crafted rocket tow floats, adding a playful twist to the sport. These innovations exemplified the pioneering spirit of the athletes.

By 2009, ÖTILLÖ attracted 50–60 teams, marking a solid foundation for the growing sport. In 2011, swimrun got its name from Erika Rosenbaum, one of ÖTILLÖ, The Swimrun World Championship podium finishers. The ÖTILLÖ Swimrun Engadin race in 2014 was the first ÖTILLÖ race outside of Sweden. It launched the international swimrun movement. Nowadays, there are hundreds of swimrun races across Europe, America, Africa, Australia, and New Zealand.^[2] To further develop the sport, in 2023 Swimrun AG, owner of ÖTILLÖ, The Swimrun World Championship, and the ÖTILLÖ Swimrun World Series, acquired a majority stake in Swimrun USA Inc., a prominent organizer of Swimrun events in the United States, called Ödyssey. As part of the acquisition, all Ödyssey races will become part of the ÖTILLÖ Swimrun World Series by 2024.

From being a single endurance challenge in its early years, ÖTILLÖ has developed into a worldwide sport, the sport of swimrun. There are now more than 600 swimrun races around the world and an estimate of 50.000 active athletes involved in the swimrun movement. Today, ÖTILLÖ is internationally recognised as one of the toughest one-day races in the world. Every year, on the first Monday in September, 160 swimrunners start the ÖTILLÖ from Sandhamn to Utö.

The ÖTILLO Swimrun World Championship course starts on the island of Sandhamn at Sandhamn Seglarhotell. On their way to the finish on Utö, the athletes will complete 24 trail running sections ranging from 0.2 to 17 kilometres and 23 swimming sections ranging from 50 to 1600 metres. They pass 24 islands, including Runmarö, Nämdö and Ornö.

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