The 37-year-old former Scotland international toured with the Lions in 1997 and 2001, winning six successive Test caps against the Springboks and the Wallabies.
He was a key part of the Lions' stunning series triumph in South Africa and was regarded as a hugely popular and committed Lions tourist.
Smith won 61 caps for his country between 1997 and 2005 and has made close to 200 appearances for Northampton since moving to Franklin's Gardens from Brive in 2001.
"There have been plenty of highlights during my career and I have worked with some great coaches and players," said Smith, who had only three Scottish caps to his name before being named as a Lion first time around.
"Obvious highlights are the 1997 Lions tour to South Africa and winning the 1999 Five Nations with Scotland.
"Being part of the '97 Lions was an incredible experience, especially working with Ian McGeechan and Jim Telfer. Working with Wayne Smith at the Saints was also very enjoyable.
"I've thoroughly enjoyed my time at Northampton. It is a community club with great fans and Franklin's Gardens has been a good home for the past eight years. I will take away some great memories such as the Heineken Cup quarter final win in San Sebastian (against Biarritz) and other big European matches at Franklin's Gardens."
Northampton director of rugby Jim Mallinder described Smith as a great role model for future generations as he paid tribute to the achievements of one of the sport's quiet men.
"Tom has been one of the best players in the world in his position and it has been a pleasure to work with him over the past two years," said Mallinder.
"What he has achieved during his career is something young players should aspire to. He has played at the highest level, won a test series with the Lions and experienced all the highs and lows that come with professional sport.
"I, like everyone here at the Saints, would like to wish Tom all the best for the future."