The 38-year-old will front the ITV competition for a sixth consecutive year, the broadcaster announced today (May 17).
"I'm delighted to be returning to The X Factor as I just love the gig," O'Leary said. "I hope The X Factor 2012 continues to bring a sense of fun to Saturday and Sunday nights and gives the nation a show that they can laugh, cry and sometimes scream at."
X Factor boss Simon Cowell added: "Dermot has done a brilliant job for the past five years - he is Mr X Factor."
ITV Director of Television Peter Fincham also said: "Dermot is a class act; we're delighted to have him as the host of The X Factor. He's a valuable part of what makes the show popular with viewers."
O'Leary's return follows the confirmation that Gary Barlow and Louis Walsh will sit on the show's panel again for its ninth series.
Last year's winning judge Tulisa Contostavlos is also expected to return, while Kelly Rowland's departure was announced in April.
Dannii Minogue, who judged The X Factor between 2007 and 2010, has been heavily tipped as her replacement.
Filmed auditions with the panel will begin next Wednesday (May 23) in Liverpool.
Watch Dermot O'Leary talking about the 2012 series of The X Factor UK below: