The princess of pop surprised the producer with a cake and his very own rendition of 'Happy Birthday'.
© Fox
Britney was accompanied by fiancé Jason Trawick as she serenaded LA in front of fellow judges Demi Lovato and Louis Walsh, who was standing in over at US X Factor for an ill Simon Cowell.
LA was clearly moved by the impressive warbling, calling Brit "the sweetest thing in the world".
© Fox
Awwwww!
US X Factor started over in the States earlier this month and we'll be getting it from Thursday, 27 September on ITV2.
Watch Brit in action here:
How do you think Britney did? Let us know in the comments below and you could win £50!
Read more: Watch Louis Walsh on US X Factor with Britney Spears!
Read more: Simon Cowell likes Britney Spears's breasts, she likes his chest hair!
