The broadcaster, who hosts an afternoon show on BBC Radio 1 Xtra, said: "I've been a huge fan of The X Factor since day one and have watched every series religiously, so to be the new host of The Xtra Factor in its most exciting year ever, is what dreams are made of.
"I'll be asking the judges the questions that people at home are screaming at their TVs - and you'd better believe I'm going in with a bang. I just can't wait to get on the road now and meet the auditionees."
The show's executive producer, Mark Sidaway, said she was "a great addition to the team".
He said: "She has fantastic knowledge of the music scene and has interviewed some of the biggest names in the business, so will be brilliant at digging out all of the gossip from the judges and contestants. She also has a great sense of humour...a must for the show."
Flack announced she was leaving yesterday, breaking the news on Twitter.
She wrote: "Heyyyyyy! After three brilliant years of hosting Xtra Factor it's time to pass the baton. It's been incredible and I'll never forget it."
She joked: "How can I when I am married to Louis Walsh? It's all he ever talks about at home.
"I wish the new team all the best and look forward to watching the new series."