Daiya Seto(瀬戸 大也,Seto Daiya?, born 24 May 1994) is a Japanese swimmer who competes in the individual medley, butterfly, breaststroke, and freestyle. At the World Short Course Championships in 2012, he won the gold medal in the 400 meter individual medley. Seto holds the Asian records in the 200 meter and 400 meter individual medley (short course).
Seto just missed out on making the Olympic team in 2012 when he finished third in both the 200 m and 400 m individual medley events. He was however able to the near miss in the past by being very successful at the 2012 Fina World Cup. He ended up competing in all the stops on the cup, and ended up coming fourth overall in the rankings. He finished off the short course by going to the 2012 Short Course World Championships. This would turn out to be the meet where Seto would win his first international medals of his career. He first competed in the 400 m individual medley, where he won his first World Championship title by coming in first with a new Asian record of 3:59.12, over a second and a half faster than second place finisher László Cseh. He then competed in the 200 m individual medley, where he was able to rally past Cseh for the silver in a time of 1:52.80, over three seconds behind the winner and a new world record from Ryan Lochte.
Despite coming in as a short course World Champion, Seto was still coming into the championships relatively unknown, mostly due to Japanese medley star Kosuke Hagino. He once again came in with the schedule of swimming both medley events. In the 200 m, his first event he swam a personal best in the semi finals with a 1:58.03, and then finished a touch slower the next night to finish seventh overall. In his final and best event, the 400 m individual medley Seto qualified first for the final and then surprised everyone when he won his first long course world champion title by finishing first in a time of 4:08.69, about half a second ahead of second place finisher Chase Kalisz. He is the second fastest Asian performer in the event, behind Hagino.