The 23-year-old "Hot" singer and model made the announcement on her social media pages on Saturday (05Sep20), offering up a photo of herself in lingerie.
"Baby, check me out. I'm finally free!! Only Fans," she captioned the post.
Both Bella and Cardi joined the site last month (Aug20) to offer fans racy photos and footage that might get them banned from other social media platforms.
Actress Thorne has already raised eyebrows after joining the subscription-based platform, best known for its adult content, breaking the record for the most earnings in one week and becoming the first account holder to bank $1 million (£760,000) in her first 24 hours.
She has also been criticised by fans who allegedly did not receive the "nude" picture she is said to have charged $200 (£150) for.
Bella has since apologised, insisting she had only joined OnlyFans as part of her research for an upcoming film role.