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Jenner, the half-sister of Kim, Khloe and Kourtney Kardashian, debuted Kylie Cosmetics in 2016 with $US29 ($NZ42) lip kits containing matching lipstick and lip liner, and has since sold more than $US630 million worth of makeup.
Forbes said it valued the 20-year-old's company at nearly $US800 million and that Jenner owns 100% of it.
Adding millions from TV programmes, endorsements and after-tax dividends from her company, Forbes said Jenner was "conservatively" worth $US900 million.
Another year of growth would make her the youngest self-made billionaire ever, male or female, the magazine said.
Jenner first grabbed the spotlight as part of the Keeping Up with the Kardashians reality TV show with her mother and siblings.
Forbes estimated that Kim Kardashian, who has her own cosmetics, clothing and mobile games lines, is worth $US350 million.
In an interview with Forbes, Jenner credited social media for helping drive her success.
"Social media is an amazing platform," she said. "I have such easy access to my fans and my customers."