Ariarne Titmus has defended her women's 400m freestyle title with an incredible swim at the Paris Olympics, completely dominating her American rival Katie Ledecky in the final. 

In what's been called the race of the century, the Tasmanian-born Titmus clocked three minutes 57.49 seconds to win from Canada's Summer McIntosh (3:58.37) and Ledecky (4:00.86). 

Titmus led early as she and McIntosh both started strongly at a pace under the world record.

'Ledecky only knows how to swim this race one way - she goes hard,' Giaan Rooney said in commentary for Channel Nine, but the American soon faded out of contention for gold.

The Aussie was in a comfortable rhythm as she slowly built her lead moving into the middle stages of the race, by which time she had briefly slipped just under the world record pace.

The record looked in danger soon after that as Titmus kept kicking away from the American with 100 metres left.

Her lead kept growing in the last stage of the contest, with Ledecky completely fading and Summer Mackintosh from Canada her only competition.

The win makes Titmus the first Aussie woman to defend an individual swimming gold since Dawn Fraser in 1964. 

The Aussie was a clear-cut winner - with famously excitable coach Dean Boxall once again going berserk in the stands after her triumph.

'It's another thing to come in and defend your title, it's a big monkey on your back,' Titmus said after the victory, adding 'I can't really believe that's me' when she was asked about equalling Fraser's achievement.

'Hopefully it's inspiration to anyone, that it doesn't matter where you come from - small town, big town.

'My legs started to go but I just had to hold on and leave everything out there.

'I'm buggered,' she said.

'But I feel relieved. It's a different feeling winning it again after the first time.

'I know what it takes to be Olympic champion and I know how hard it is racing in these circumstances at an Olympic Games.

'It's not really like anything else, the noise and atmosphere and pressure.'

Australia's Jamie Perkins finished last in the race.

Titmus joins Fraser as the only Australian women to successfully defend an Olympic swimming individual title. Fraser won 100m freestyle gold at the 1956, 1960 and 1964 Olympics. Shirley Strickland had previously accomplished the feat in athletics.

The Tasmanian, who is the world record holder in both the 200m and 400m freestyle, is also the reigning Olympic 200m freestyle champion - heats and semi-finals of that event will be contested on Sunday with the final on Monday.

Titmus's triumph followed fellow Australian Elijah Winnington claiming the silver medal in the men's 400m freestyle, though teammate Sam Short finished fourth.