Hagino made his big 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 was able to pick up his first World Championships medal with a silver and a new Japanese record of 3:44.82. He would be back in finals action on day 3 of the meet of where he competed in his first final of the night, the 200 m freestyle. Despite being in medal contention, Hagino was caught in a close race and was locked out of the medals in fifth place with a new personal best time of 1:45.94. Nearly an hour later, and he came to swim the final of the 100 m backstroke and place seventh in 53.93, much slower than his Japanese nationals time 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 Japanese nationals time of 1:55.74, but it was his second medal of the meet. The following day he would lead off Japan for the 4 x 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 also placed fifth in the 200 m backstroke final that night, finishing in 1:55.42. On the final night of competition, despite coming in as the favourite, Hagino was again only able to manage fifth place, finishing in 4:10.77. Although only winning two medals in his seven eventsf, he was the only swimmer at the meet to swim six individual events.