The princess of pop surprised the producer with a cake and his very own rendition of 'Happy Birthday'.
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".
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:
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