Australian celebrities have praised swimmer Ariarne Titmus after she won gold at the Tokyo Olympics on Monday.

The 20-year-old triumphed in the women's 400m freestyle, coming from behind to defeat American five-time Olympic champion Katie Ledecky.

Stars including Bec Judd, Pia Whitesell, Bianca Chatfield and Em Rusciano reacted to Titmus' victory with joy on social media.

Bec, 38, shared a photo of the swimmer to Instagram and wrote: 'Amazing. How about that last 100m.'

'Yassssss!!' comedian Em, 42, commented.

Former Home and Away star Pia, 37, also shared a picture of the race and added a love-heart emoji.

Meanwhile, blogger Sophie Cachia wrote, 'Woooo crying,' alongside a party emoji.

The Block star Bianca Chatfield also commented: 'What an incredible swim.'

Titmus won the gold in the women's 400m freestyle in Tokyo on Monday.

She defeated Katie Ledecky, who is regarded as the greatest female swimmer ever.

Titmus' achievement is Australia's second gold medal of the Tokyo Games, following the women's 4x100m freestyle relay success.

Coach Dean Boxall couldn't contain his excitement as he erupted into applause, tearing off his face mask and almost throwing it into the stands.