Olympic champion swimmer Mack Horton has announced his retirement from the sport, less than 200 days out from the Paris Olympics.

The 27-year-old said "the hunger wasn't there" to chase a third-straight Olympics appearance in Paris.

"I dearly wanted to swim in Paris but the hunger wasn't there," Horton said.

"I always want to give my all and I am not someone who just wants to make up the numbers, so this is the right time to step away.

"I have felt so privileged to represent Australia and wear the green and gold … I just hope Australia thinks I did them proud.

"I am so grateful for my time in swimming and in regard to legacy, I hope my teammates and the sport think that I was able to help them and the sport in some way."

Horton won the 400 metres gold medal at the Olympics, as well as titles at the World Championships and Commonwealth Games, but may be remembered most for the silver medal he won at the 2019 World Championships in Gwangju.

Finishing behind China's Sun Yang, Horton refused to shake the Chinese athlete's hand or even stand on the podium alongside him.

Sun, a two-time gold medallist at the 2012 Games in the 400 and 1,500 metres, had been banned for three months for a doping violation in 2014 after testing positive for a heart medication that had only recently been added to the WADA banned list and has subsequently been downgraded.

Horton, a fierce campaigner for clean sport, called Sun a "drug cheat" at the Rio Games and subsequently beat him to the gold medal.

In 2020, Sun was issued an eight-year ban, reduced for four years on appeal, for tampering with the doping control process by having a member of his team smash a vial containing his blood.

Dolphins head coach Rohan Taylor described Horton as "a person of great influence" and a "just a quality person".

"From a performance point of view, he was consistent and confident … in 2019 at the World Championships he didn't have a great heat swim in the 4x200m freestyle relay but he was determined to correct this and anchor the final.