In 2021, Hayden won the 50 m freestyle at the Canadian Olympic Trials with a time of 21.82, well ahead of the FINA "A" standard of 22.01. This was his fourth time representing Canada at the Olympics. He opted not to contest the 100 m in Tokyo, as he and his coaches feared it might trigger back spasms. Hayden's return was widely credited for providing leadership to the men's program, with the most notable result being the surprise fourth-place finish in the 4x100 m freestyle relay where they were just 0.3 seconds behind the Australians who took the bronze medal, an event where the Canadian team had not been expected to even qualify for the final. Hayden swam a time of 47.99 in the leadoff of the relay, becoming by five years the oldest man to ever swim a time under 48 seconds. Hayden said afterward "this means that I made the right decision to come back." Hayden also competed in the 50 Metre Freestyle where he reached the semi-finals and just missed a finals slot by 0.04 seconds, nonetheless it was his fastest time (post bodysuit), and his best-ever time in the event, leaving him to write on social media “It is such a weird state of confusion when you are torn between the pure competitor in you who realizes you missed the finals by 0.04 seconds and the human who realizes that … at 37 years of age, I came within only 0.04 seconds from the Olympic final.”.