O’Leary, 46, added: “We’re pleased to announce that we’re expecting a little Koppang O’Leary.”
His wife, TV producer Dee Koppang O’Leary, 41, shared the same message on her Instagram page.
Shortly after they shared the news, the pair were photographed together attending a party for Bafta nominees at Kensington Palace ahead of the awards ceremony on Sunday.
The pair married in September 2012.
The couple were inundated with congratulatory messages on social media.
TV presenter Vernon Kay said: “Welcome on board team K. O’L!
“Congratulations from the Ks!”
O’Leary said he was “pleased” about the news (Ian West/PA)
Singer Sophie Ellis-Bextor wrote: “That’s so lovely.
“So happy for you both (and happy for your baby, too!)”
O’Leary previously told The Sun in 2015 that him and his wife were too busy with work to have children.
He told the newspaper: “I do want kids with my wife, but I want them when we both think it’s the right thing to happen.”
Dermot O’Leary and TV producer wife share baby joy news