The broadcaster stepped down from his role at ITV News last year, saying that he made a “misjudgment which I regret”.
He said GB News, a new network chaired by broadcaster Andrew Neil, is an opportunity to “challenge the groupthink”.
Other signings include broadcaster Kirsty Gallacher, ex-BBC presenter Simon McCoy, the former executive editor for The Sun, Dan Wootton, and ex-Sky broadcaster Colin Brazier.
Stewart said: “GB News is a unique opportunity to broaden the spectrum, challenge the groupthink and shift away from a predictable agenda in news and debate.
“I can’t wait to get stuck in and tackle issues that really matter to people across the United Kingdom.”
Director of news and programmes John McAndrew said: “Alastair is hugely respected across the country and we are honoured to have a journalist of his calibre and experience on board. He will anchor GB News in more ways than one.”
— GB News (@GBNEWS) April 2, 2021
A Twitter exchange, in which he quoted a Shakespeare passage, including the phrase “angry ape”, is thought to have led to his departure.
The broadcaster later told The Sunday Telegraph: “The decision that I took very clearly back in January, was to draw a line and not sit in my lovely farmhouse in Hampshire and weep and do the sums of my pension.
“I said ‘OK, fine, what else can I do?’, I’ve got the experience that I’ve got. Nobody can take that from me. I’m the age that I am, I can’t do anything about that. But there might be something to offer.”