Paris, France - A dominant Ariarne Titmus held off Summer McIntosh and Katie Ledecky to successfully defend her Olympic 400m freestyle title Saturday, leading from start to finish.

The Australian world record holder was in full control to touch in 3mins 57.49secs ahead of Canada's McIntosh (3:58.37) and decorated American Ledecky (4:00.86), who faded badly in the final 50m.

"Oh, my legs are a bit tired, but I'm just relieved more than anything," said Titmus.

"I probably felt the expectation and pressure for this race more than anything in my life, to be honest, and I'm pretty good at handling the pressure, but I've definitely felt it.

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"I'm just happy to get the result for myself, and I feel so honored to be a part of the race and be alongside legends like Katie."

Titmus, who dethroned Ledecky in Tokyo, came to France in red-hot form after shattering the 200m world record and swimming the second-quickest 400m ever at the Australian trials.

But Ledecky can never be written off, and McIntosh was always going to be a threat after snatching Titmus's world record last year before the Australian took it back.

However, it was all Titmus in Paris, who was near world record pace at the halfway mark and comfortably surged home with McIntosh hauling in Ledecky in the final lap to win her first Olympic medal.

Titmus will also swim the 200m and 800m in Paris.

Ledecky has opted out of the shorter distance, focusing instead on defending her 800m and 1500m titles.

McIntosh is also skipping the 200m free, instead prioritising the 200m and 400m medley and 200m butterfly.