Khloe Kardashian & Mario Lopez -- Officially Sign On to 'X Factor'

It's official ... Khloe Kardashian and Mario Lopez will be the new co-hosts of "X Factor" ... TMZ has confirmed with sources connected to the show.

We broke the story ... Khloe and Mario had been thiiiiiiis close to a deal for months ... but now we've confirmed they've both signed on the dotted line ... it's a done deal.

We're told the duo's salaries will add millions to the budget -- on top of an already enormous talent budget ... with Britney Spears in the lead at $15 mil.