Justin Bieber has praised Simone Biles for prioritising her mental health over the Olympic Games.

The American gymnast pulled out of two finals at the Tokyo sporting event on Tuesday, and explained to reporters that she wasn't in the right frame of mind to compete and had been "having a little bit of the twisties" - a psychological phenomenon where a gymnast loses air awareness while performing.

Following Simone's announcement, Justin took to Instagram to voice his support for the 24-year-old athlete.

"Nobody will ever understand the pressures you face! I know we don't know each other but I'm so proud of the decision to withdraw (sic)," he wrote. "It's as simple as - what does it mean to gain the whole world but forfeit your soul. Sometimes our no's are more powerful than our yes's. When what you normally love starts to steal your joy it's important we take a step back to evaluate why.

"People thought I was crazy for not finishing the purpose tour but it was the best thing I could have done for my mental health!! So proud of you @simonebiles."

A number of other celebrities have also sent Simone, who is the most decorated American gymnast, kind messages.

"Yes. This is what a champion looks like. So proud of #SimoneBiles and the entire gymnastics team. How lucky we are to have you all as our Team USA," wrote actress Uzo Aduba, while TV host Hoda Kotb added, "Someone said it best. @Simone_Biles already won. She is a class act. Withdrew from team competion after vault... stayed and cheered on her teammates... got them chalk for their hands.. encouraged.. hugged them. She already won."