Matthews said he was taking time out to "look outside for my next big challenge in communications". His role at the Omnicom-owned Fishburn will not be replaced directly.
The former CEO of Fishburn Hedges has been in the chairman role since the agency emerged in its current form in 2014, from the three businesses of the Fishburn Hedges Group. He joined the group as its first account manager more than 20 years ago.
Matthews said: "Fishburn is in my blood and always will be. But, however much I love it here, I’ve reached the point where I’ve literally done every job in the business and now is a natural point to take a break and look outside for my next big challenge in communications. So I’ll be taking some time out over the summer to have the sabbatical I’ve never taken and to do exactly that.
"Fishburn is led by a fantastic, new management team, with the brilliant Ali [Gee, CEO] at the helm. We have some great young talent coming through, which combines the very best values and heritage of the business with a new sense of energy and direction. It is perfectly set to deliver real change for clients in an exciting new world of communications and I have every confidence the team will take the business to new heights."
Gee said: "We owe Simon an enormous debt of gratitude. He has dedicated himself to our business for over two decades. He has led our people and our clients on a journey that has seen Fishburn do amazing work for great businesses and brands.
"But more than any of that, Simon has been the load-stone for our agency culture. He has been friend, counsel and coach to literally hundreds of Fishburners in his time and I know every one of them is grateful."
In February, PRWeek reported that Fishburn had made four redundancies in its consumer team.
Fishburn was ranked 25th in PRWeek’s Top 150 PR Consultancies report for 2015, with an estimated annual fee income of £12m.