Four-time Olympic gold medallist Ariarne Titmus announced her immediate retirement from swimming on Thursday, calling it a "really tough" decision as she was lauded as "a living legend".

The 25-year-old Australian had been on an extended break since the Paris Olympics but had previously said she planned to return for the 2028 Los Angeles Games.

"A tough one, a really tough one, but one that I'm really happy with," she said on Instagram. "I've always loved swimming, it's been my passion since I was a little girl.

"But I guess I've taken this time away from the sport and realised some things in my life that have always been important to me are just a little bit more important to me now than swimming, and that's okay."

Titmus won the 400m freestyle titles at the Tokyo and Paris Olympics, beating American great Katie Ledecky each time in what were dubbed the "races of the century".

In doing so, she became the first Australian swimmer since Dawn Fraser in 1964 to clinch back-to-back gold medals in the same event.

Known as "the Terminator", Titmus lost her 400m world record to Canadian phenomenon Summer McIntosh earlier this year, but remains the fastest women ever over 200m.

"An outstanding competitor, champion and person! Congratulations @ariarnetitmus," Ledecky said in reacting to her retirement, while McIntosh said: "You will be missed."

"I always intended to return," said Titmus. "I never thought that Paris would be my last Olympic Games. And knowing now what I know, I wish I had of maybe enjoyed that last race a little bit more."

Before Paris, Titmus underwent surgery to remove a benign ovarian tumour and said the health scare "really rocked me".

"A time came for me when a switch was flicked, it was in the lead up to the Paris Games. I went through some health challenges, which, quite frankly, really rocked me mentally," she said.

"It probably was the first time where I considered some things outside of swimming. My whole swimming career, I've been all or nothing, and that's how I've had to be to become the athlete that I am. 

"But I think delving more into those health challenges, I've really had to look within and think about what's most important to me and beyond swimming."

Titmus hangs up her goggles with a haul of 33 international medals, including eight at the Olympics, nine from world championships and eight at the Commonwealth Games.

"You are incredible. Privileged to have witnessed a living legend in the sport," said fellow Australian and five-time Olympic gold medallist Kaylee McKeown.

Mollie O'Callaghan, the Olympic 200m freestyle champion, said she was grateful to train alongside her while sprint king Kyle Chalmers added: "I'm so proud of you. You are a legend in and out of the pool."