Britney Spears -- Big 'X Factor' Announcement

It's the biggest TV news in a long time -- Britney Spears officially signing on to judge the next season of "X Factor" for $15 MILLION -- but Brit looked nervous during the big announcement today.

Simon Cowell made the announcement moments ago at the Upfronts in New York -- publicly unveiling the two newest judges on "X Factor" ... Britney and Demi Lovato.

But unlike Demi, Britney sort of pulled a poker face when Simon called her name.

And when Britney briefly spoke, she sounded nervous, saying, "I am so excited about this whole experience. It's gonna be so much fun and so different from anything I've ever done and I'm ready to find the true star." Demi added, "I am totally stoked to be here!"