The 34-year-old, who joined the ITV programme in 2011, broke 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."