Barrie, who has been editor of the Plymouth-based newspaper since 1995, will step down from his role on July 31.
Following his departure, Alan Qualtrough, editor of the Evening Herald in Plymouth, will take on the new role of editor-in-chief of the two titles.
He will continue to edit the Evening Herald and the new editor of the Western Morning News will report to him.
Barrie is the third Northcliffe editor to leave in the past three weeks.
Mike Lowe quit the Bristol Evening Post last week, to be succeeded by Mike Norton who will be editor-in-chief in a new role covering the Post and Western Daily Press. The week before, David Gledhill stood down from the Bath Chronicle.
Prior to joining the Western Morning News, Barrie edited the Nottingham Evening Post for 14 years and the Kent Evening Post for five years.
He presided over the successful transformation of the Western Morning News to tabloid format and steered the newspaper through a difficult period in the region’s history when it was hit by the foot and mouth crisis.
Under his editorship the newspaper has won a string of awards, including Press Gazette’s Daily Newspaper of the Year for both 1997 and 1998.
In 1998 the Western Morning News also won Press Gazette’s Overall UK Newspaper of the Year award, which followed Barrie’s achievement of winning the same title for the Nottingham Evening Post in 1991.
This year the NFU Countryside organisation voted the Western Morning News UK Rural Newspaper of the Year, and this month the title was chosen as EDF Energy South West Daily Newspaper of the Year.
Barrie was last year also honoured personally by the Prince of Wales, when he was made an Ambassador for the West Country in recognition of his work through the Prince’s Rural Action Group.
Northcliffe Newspapers managing director Michael Pelosi said: “Barrie is a formidable campaigner for the West Country and I take this opportunity to thank him for all that he has done for the Western Morning News and for the West Country during his editorship.”
Before joining Northcliffe in 1991 Alan had been news editor of the Daily Express, and he began his career at the Liverpool Daily Post.