The Keeping Up with the Kardashians star and the Extra presenter officially announced their involvement in the Fox talent show today (October 16).
In a message on her official website, Kardashian wrote: "Hi loves. I'm sure you've been hearing the rumours and I'm SO excited that I can finally confirm the news - I am joining The X Factor as co-host with Mario Lopez!!!!
"I'm a huge fan of the show so I literally fell over in my chair when I heard that I was being considered for the role. The show already has such an incredible panel of judges and it is truly an honour to have been given this opportunity to work with them.
"I'm thrilled, overjoyed and of course a little nervous about this new step in my career, and I can't wait to get started. Wish me luck!"
Former Saved by the Bell actor Lopez added on Twitter: "Alright guys, it's official… I'll be hosting @TheXFactorUSA ! Looking forward to working with @SimonCowell and the rest of the amazing team."
In a statement, Lopez went on to say: "I'm very excited to be joining the X Factor team. I love the show, and I've been a big fan of Simon Cowell and the judges for a long time. Khloe and I have been friends for years and I know we'll be a great team and have a blast."
Cowell joked: "The worst kept secret in Hollywood is out. Mario and Khloe are our hosts. They will debut on our first live show in November and I couldn't be happier."
The pair take over from the series' first season presenter Steve Jones.
Both Kardashian and Lopez have been linked to the hosting vacancies for weeks.
The X Factor USA's live shows will begin on Thursday, November 1 on Fox, following the conclusion of the Major League Baseball World Series. The show airs on ITV2 in the UK.
Watch a promo for this week's X Factor USA - airing on Wednesday (October 17) at 8/7c on Fox - below: