Britney Spears sings 'Happy Birthday' live on 'X Factor' USA - video
The singer serenaded fellow judge LA Reid with the song on the show to celebrate his 56th birthday.
The footage airs on tonight's episode of the series (September 19), which also features a guest spot from The X Factor UK judge Louis Walsh.
It was recently reported that Spears braved temperatures of almost 38 degrees to film her latest segments for the show in California.
Last weekend, the star threw a birthday party for her sons Jayden James and Sean Preston, hiring a trampoline park and Sky High arcade for the occasion.
Watch Spears on The X Factor USA below: