Four-time Olympic gold medallist, Ariarne Titmus, has announced her retirement from swimming.

Titmus announced the news on Instagram in a heart-warming letter to her seven-year-old self.

“Dear seven year old Ariarne, today you retire from competitive swimming,” she wrote.

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Ariarne Titmus.  Getty

“18 years you spent in the pool competing. 10 of those representing your country. You went to two Olympic Games and ever better, you won!!!

“The dreams you had .. they all came true. The friends you’ve made .. they’re for life. You achieved more than you ever thought you were capable of and you should be so proud.”

The Instagram post included a video where Titmus admitted it was a “tough” decision.

“I love swimming, I’ve always loved swimming, it’s been my passion since I was a little girl but I guess I’ve taken 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 OK,” she said.

Since the Paris Olympics, Titmus has been on a hiatus from competitive swimming but such was the secrecy of her decision, there has been no speculation that she would retire.

Titmus has been working for Channel Nine during her absence, providing colour commentary at the Australian Open and has recently been announced on the latest show ‘Shark!’

The 25-year-old admitted she initially did intend on returning to swimming and hadn’t intended on Paris being her last Olympics.

In 2023, prior to Paris, Titmus underwent ovarian tumour surgery, which played a massive role in her decision to retire.

“Knowing now what I know, I wish maybe I enjoyed that last race a little bit more,” she said.

“But I guess having this 12 months away I’ve had the chance to explore what life is like without swimming and that was always my intention but I think a turning point for me was in the lead up to the Paris games I went through some health challenges which quite frankly, really rocked me mentally.

“It probably was the first time where I really 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, I’ve just been in this ruthless pursuit for my goals and working alongside Dean (Boxall) that’s how we both decided it had to be for me to beat Kate Ledecky.

“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 I’ve always had goals in my personal life but swimming’s always been most important up until this point and I’ve realised those goals and what I want in my future is now more important for me.

“More than anything, I’m excited for what’s next, I’m excited I have this opportunity to delve into life and other things that I’m wanting to chase and look forward to.”

Titmus went on to say her first gold medal in Tokyo, against Ledecky in the 400m freestlye, will go down as her greatest achievement.

“Without facing Katie, I wouldn’t have been the athlete I am. She set this incredible standard and I was so willing to chase it,” she said.

“I’m so grateful I’ve been able to part of such an incredible rivalry, we both have so much respect for each other.”

Titmus finishes her career with 33 international medals, which includes eight Olympic medals (four goal, three silver and one bronze).

She still holds the 200m freestyle world record.

At Paris, Titmus defended her 400m freestyle gold medal against Ledecky and Canadian prodigy Summer McIntosh.