[1][4] He narrowly missed the 2012 Japan Olympic Team when he finished third in both the 200-metre and 400-metre individual medley events at the national selection meet.

He first competed in the 400-metre individual medley, where he won his first world title in a new Asian record of 3:59.12, over a second and a half faster than second-place finisher László Cseh.

[7] Earlier in the meet, he did not live up to the expectations of others in the 200-metre butterfly and 200-metre individual medley, events in which he was ranked second in the world before leading up to the Championships.

[8][9] Seto earned a bronze medal in the 400-metre individual medley on 6 August, with a time of 4:09.71 in the final of the event at the 2016 Summer Olympics in Rio de Janeiro, Brazil.

[25] The same day Seto was announced overall male winner for the Doha stop, SwimSwam revealed he had officially become a professional swimmer.

[26] On the first day of competition, 28 October, at the fourth and final stop of the World Cup circuit, held in the Palace of Water Sports in Kazan, Russia, Seto set new Asian and Japanese records in the 100-metre individual medley with a time 51.29 seconds.

[31][32] Seto snuck into the final of the 200-metre breaststroke on the last day of competition of the 2021 World Cup circuit, ranking fifth overall with a time of 2:05.83 in the prelims heats.

[39] In December 2021, Seto was announced as one of two swimmers from Japan to represent the country in competition at the 2021 World Short Course Championships in Abu Dhabi, United Arab Emirates.

[41][42] In the evening final, he won the gold medal with a time of 1:51.15, finishing two-tenths of a second ahead of silver medalist Carson Foster of the United States.

[47] Day five of competition, he ranked first in the prelims heats of the 400-metre individual medley, qualifying for the final over one second faster than second-ranked Carson Foster with a time of 4:00.84.

[50] Two days later, he won the silver medal in the final of the 200-metre butterfly, finishing 0.95 seconds behind gold medalist Chad le Clos in a time of 1:49.22.