During Barwell's time as chairman the Saints won the Heineken Cup, European Challenge Cup and LV= Cup.
In 2009 Mr Barwell was honoured with an OBE for services to Northampton.
"I have had the fortune of being involved in club rugby during a very exciting time and it has been an honour being chairman of the Saints," Mr Barwell said.
"I have worked with a superb board of directors and some excellent directors of rugby and coaches. It has been a roller-coaster ride, with both silverware and relegation, and every minute of it will live long in the memory.
"Although I have been chairman, I am first and foremost a Saints fan and have had the privilege of watching some world class players wear the black, green and gold. It has also been an honour to serve the Saints fans, who put in considerable amounts of time and money supporting the club, and we have had an excellent relationship, as I have had with the staff at the club.
"I believe that it is the right time for me to be stepping aside as chairman. The club is on an upward trend on and off the field and will be in safe hands going forward."
Although he is standing down as chairman, Mr Barwell will continue on the club's board of directors for a further 12 months with the Barwell family remaining as majority shareholders.
Mr Barwell's son, Leon, will take over as chairman in September, following a unanimous vote of the Northampton Saints board in early March. A director of The Barwell Corporation and VSG, a nationwide security company, Leon Barwell has been on the Saints' board since 1999 and has been involved in a wide range of projects, including the initial public offering and formation of Northampton Saints plc and the redevelopment of Franklin's Gardens.
"It's a huge honour for me to be able to play such an active role in a club that means so much to me," said Leon Barwell.
"As a family we're very proud of what's been achieved at the Saints over the past 15 years, and one thing you know for sure is that we have to go as far again in the next 15 if we want to stay at the top of the game.