The TV presenter and journalist, 62, will host a new programme in the new year, GB News announced.
His wife and on-screen presenting partner, Ruth Langsford, will continue to work with ITV as an anchor on Loose Women and a member of the This Morning team.
Holmes said: "I've spent my career on broadcasting firsts and start-ups and GB News is one of the most exciting yet. It's just the kind of shake-up the industry needs.
"I've admired GB News from the beginning for its clever mix of punchy debate but delivered with warmth and even some fun.
"To me the greatest honour in journalism is to give a voice and respect to the unheard, and that's exactly what GB News is all about."
A spokeswoman for ITV said: "We would like to wish Eamonn all of the very best in his new role.
"As a duo, Eamonn and Ruth have been part of the This Morning presenting team for the last 15 years and we thank Eamonn for all of his hard work and contribution to the show.
"Ruth remains an important part of ITV Daytime both as a continuing member of the This Morning family and as an anchor presenter on Loose Women."
Alongside his wife, Holmes has been a regular fixture on This Morning for 15 years.
However, in November 2020, ITV confirmed the duo would be leaving the Friday show, with Dermot O'Leary and Alison Hammond taking their place.
They have continued to host the daytime programme during the holidays.
Before This Morning, Holmes presented GMTV and has also worked across radio, with slots on stations including Radio 5 Live, Magic, BBC Radio 2, and talkRadio.
Holmes, from Belfast, is the latest signing to fledgling channel GB News.
He joins on-air talent including former Sky anchor Colin Brazier, ITV News journalist Alastair Stewart, BBC journalist Simon McCoy and former Labour MP Gloria De Piero.
In November, the broadcaster Stephen Dixon also announced he was joining the channel after nearly 22 years with Sky.
The news comes two months after former BBC political broadcaster Andrew Neil stepped down as the chairman and host of a prime-time show on GB News.