The 42-year-old broadcaster said he is “excited to be back” as presenter.
X Factor boss Simon Cowell added: “Dermot’s back, hooray! And so is his dancing! Welcome home, Dermot.”
O’Leary, who had been a mainstay of Cowell’s show for eight years, departed in 2015 saying it was “time for me to move on”.
However, the star has now said he is “really looking forward” to the next instalment.
“I’m very flattered to be asked back to The X Factor, and am currently dusting off my dancing shoes,” O’Leary stated.
Olly Murs and Caroline Flackat took over last year.