Kosuke Hagino (萩野公介, Hagino Kōsuke^?, born 15 August 1994) is a Japanese competitive swimmer who specializes in the individual medley and 200 m freestyle. He is a four-time Olympic medalist, most notably winning gold in the 400 m individual medley at the 2016 Summer Olympics.

Hagino holds the Asian Records in the 200 m and 400 m individual medley (long course), the 100 m and 200 m individual medley (short course). With team Japan, he holds the Asian Records for the 4×100 m freestyle relay and the 4×100 m medley relay.

Having attends Toyo University, and is coached by Norimasa Hirai. He is one of the only two Asians to be voted World Swimmer of the Year.

Hagino made his international breakthrough at the 2012 Olympics held in London. Hagino qualified First in the 400 m individual medley heats with a new Asian record of 4:10.01, and would go on to win his first international medal with a bronze in the event and again lower his Asian record to a 4:08.94.

Coming into the Championships Hagino had qualified for a full slate of events including the 200 m freestyle, 400 m freestyle, 100 m backstroke, 200 m backstroke, 200 m individual medley and the 400 m individual medley. In his first event the 400 m freestyle Hagino won his first silver medal at the World Championships medal with a new Japanese record of 3:44.82.

In the Finals of 200 m Free Style, Hagin clocked a personal best time of 1:45.94.; he came in 5th.

Nearly an hour later, he was swimming, this time in the Finals of the 100 m backstroke. He was placed seventh in 53.93, much slower than his National record of 53.10 (which would have won him a silver medal.)

On day five, after qualifying for the final, Hagino won another silver medal in the 200 m individual medley. His time of 1:56.29 was about half a second off his Nationals time of 1:55.74; he won Silver The following day, he led off his team, in the 4 × 200 m freestyle relay, and was able to take off a hundredth of a second off his 200 m free time from day three, swimming a 1:45.93. He was placed fifth in the 200 m backstroke final that night, finishing in 1:55.42.

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