She is said to have signed a £1 million deal to take the last spot in the new judging panel, alongside Simon Cowell, Louis Walsh and Cheryl Cole.
Mel B, 39, has previously appeared as a guest judge on the panel, as well as on both the American and Australian versions of the show.
The deal was agreed in Sydney, where the singer is currently a judge on talent show The Voice Kids.
Her £1 million fee includes a £100,000 wardrobe and expenses allowance.
Cheryl Cole, 30, and show boss Simon Cowell, 54, revealed in March that they would revive their on-screen partnership for the ITV talent show after a four-year break.
Cole said at the time that she was hoping for a "strong woman" to be added the to panel of four.
The first X Factor auditions will start in Manchester on Monday, reportedly a day later than planned to accommodate Mel B.