Ariarne Elizabeth Titmus OAM (born 7 September 2000) is a retired Australian swimmer. She is the reigning Olympic champion in the women's 400-metre freestyle, having won the event at the 2020 Summer Olympics and the 2024 Summer Olympics and the world record holder in the long course 200-metre freestyle. Titmus is widely considered one of the greatest middle-distance swimmers of all time. In 2019 and 2020, she competed representing the Cali Condors in the International Swimming League.

In 2015, when Titmus was 14 years old, she and her family, including father Steve Titmus, moved from Tasmania to Queensland for better training opportunities. She initially attended secondary school at St Patrick's College Launceston before switching to St Peter's Lutheran College in Brisbane. Titmus first trained as a swimmer at Launceston Leisure and Aquatic Centre. She is coached by Dean Boxall, who formerly coached Stephanie Rice and Leisel Jones. Titmus is a supporter of the Hawthorn Hawks in the Australian Football League.

In December 2016, Titmus made her Australian team debut at the World Championships (25 m) in Windsor, Ontario, Canada. Her first event was the 800 m freestyle, where she finished fourth in a time of 8:17.95. She then competed in the 400 m freestyle, finishing sixth in a time of 4:02.70. Her final event was the 4 × 200 m freestyle relay, in which she split 1:55.14 on the third leg; Australia finished fourth in a time of 7:40.00.

In July 2017, Titmus competed at the World Championships in Budapest. Her first event was the 400 m freestyle, where she came fourth in a time of 4:04.26. She then competed in the 200 m freestyle, finishing seventeenth in a time of 1:58.79. She then swam in the 4 × 200 m freestyle relay, splitting 1:56.61 on the anchor leg; Australia won the bronze medal in a time of 7:48.51. Titmus' final event was the 800 m freestyle; she finished fourteenth in a time of 8:37.10.

In December 2017, Titmus competed at the Queensland Championships in Brisbane. She went 4:02.86 in the 400 m freestyle, breaking Jessica Ashwood's Australian record of 4:03.34 from 2015.

In February–March 2018, Titmus competed at the Australian Trials on the Gold Coast. She went 4:02.36 to set another Australian record in the 400 m freestyle.

In April 2018, Titmus competed at the Commonwealth Games on the Gold Coast. She won the silver medal in the 200 m freestyle in a time of 1:54.85, 0.04 behind Canada's Taylor Ruck. Titmus then competed in the 4 × 200 m freestyle relay. She split 1:55.59 on the anchor leg, securing the gold medal in an overall time of 7:48.04; Australia broke the games record of 7:49.90 set by the nation in 2014. Titmus then swam in the 800 m freestyle, winning the gold medal in a personal best time of 8:20.02. Her final event was the 400 m freestyle, which she won in a time of 4:00.93. Titmus' time was a new games record, surpassing the mark of 4:04.47 set by New Zealand's Lauren Boyle in 2014, and was also a new Australian record.

In August 2018, Titmus competed at the Pan Pacific Championships in Tokyo. Her first event was the 800 m freestyle, in which she won the silver medal in a time of 8:17.07, breaking Ashwood's Australian record of 8:18.14 from 2016. Titmus then competed in the 4 × 200 m freestyle relay, splitting 1:55.27 on the first leg to give Australia an early lead. Australia won the gold medal in a time of 7:44.12, which broke the national record of 7:44.31 from 2008, and the championship record, set by the US in 2014, of 7:46.40. Titmus' final event was the 400 m freestyle. She won the silver medal in an Australian record time of 3:59.66, becoming the third woman to break the four-minute mark in the event.