Seoul will turn its mountains, fortress walls, riverfront and downtown streets into a single endurance course this weekend as 2,000 runners from Korea and abroad take part in the 2026 Seoul International Ultra Trail Running Race, known as Seoul100K. The seventh edition of Seoul 100K runs Saturday and Sunday, with 170 international participants from 36 countries joining professional athletes, running clubs and residents across 100K, 50K and 20K events. The 100K and 50K races start at 5 a.m. Saturday from Gwanghwamun Square, while the 20K race begins at 7:30 a.m., sending runners toward destinations including Mount Inwang, Mount Bukhan, the Han River and Cheonggye Stream. Organizers have positioned the race as an urban trail-running event that combines Seoul's natural landscapes with its historic and modern cityscape, with the 100K course allowing runners up to 31 hours to complete the route. Among the international competitors are Sweden's Petter Engdahl, Switzerland's Jasmin Nunige and Germany's Marcel Höche in the 100K, while Norway's Nora Serres, Britain's Holly Page and American Step