Stewart had been responsible for guiding the paper through a painful retrenchment to help balance the books amidst continued circulation declines and had been expected to lead a celebration of its bicentenary in just three weeks time.
In a statement the National Union of Journalists said: “Ian has guided The Scotsman titles through extremely turbulent times, but always working with the union to limit damage, despite the savage cuts of recent years.
“That he has done so while managing to retain the confidence and admiration of his staff is testament to his management abilities and fair mindedness as well as his professional approach, unflappable under pressure. He will be remembered for his staunch defence of the journalism and all products he managed.”
Publisher Johnston Press has implemented a widespread programme of job losses and budget cuts in recent years, as well as implementing a full redesign of its print and digital outlets in a bid to convince remaining readers to stay the course.
Stewart will remain in post at The Scotsman until March.