He will step down from ITV's management board and hand over his line management responsibilities on 23 December 2009. He will be taking a sabbatical in 2010.
Gallagher has agreed to maintain a watching brief on public policy matters until the dissolution of Parliament, expected in March 2010.
Ruth Settle will step up to the new role of director of communications, taking lead responsibility for corporate public relations and line management responsibility for programme publicity, and internal comms.
She will report to chief operating officer John Cresswell.
The public affairs division, led by Nigel Warner, will now report to director of group development and strategy Carolyn Fairbairn.
Gallagher said of his departure: ‘It's been a fascinating couple of years at ITV. But after sixteen years on the public affairs and corporate campaign trail, and with a new leadership arriving at ITV, this is as good a time as any to move on.'