Demi Lovato Signs on to Judge 'X Factor'

Demi Lovato has officially signed her deal and will be a judge on the next season of "The X Factor" ... TMZ has learned.

According to our sources, Demi will be in NYC tomorrow for the FOX Upfronts to announce her joining the show.

Demi is scheduled to tour this summer, but our sources say her "X Factor" schedule shouldn't interfere with her touring.

Lovato should have company tomorrow ... as TMZ previously reported, our sources say Britney Spears will be in NYC to announce she's joining the show as well.