They will replace Michael Buble as hosts of next month's show after the Canadian singer pulled out to care for his ill three-year-old son.
The Brit Awards take place at the O2 in London on 22 February.
"The Brits is an iconic show that I've watched and attended many times," said X Factor presenter O'Leary.
Emma, who is currently presenting The Voice UK, said: "To present the Brits is an honour but to be doing it alongside one of my absolute favourites, Mr Dermot O'Leary is just bonkers. Bring on the Brits 2017."
Robbie Williams, Little Mix, The 1975 and Emeli SandÃ© have already been confirmed to perform at the event.
Dermot O'Leary and Emma Willis are to present this year's Brit Awards, organisers have announced.