The 'Toxic' singer, who was rumoured to be considering joining the show for a number of weeks, has inked a contract and will appear on the competition's second season alongside Simon Cowell and LA Reid, reports E! Online.
"The contract is signed. There's been a lot of back and forth over the past few weeks as they negotiated the small details, but she's on. It's all completed and Britney is beyond excited," a source explained.
"She's ready for this. She can't wait to sit down next to Simon at the judges' table and give this everything she's got. She is excited about showing the world that she can do this, and do this well. This is the perfect next stage in her career. Both Britney and Simon are very excited to work together."
Spears will reportedly receive $15 million (£9.3m) for the season.
Earlier this month, it was suggested that Demi Lovato will fill the vacant fourth role at the X Factor USA judges' table, though she has not been confirmed.
Chaka Khan has also reportedly expressed an interest in joining the show.
Watch Britney Spears's video for 'Criminal' below: