The 33-year-old still had a year left to run on his contract but has instead departed Franklin's Gardens by mutual consent.
It is thought that Spencer is keen to remain the Guinness Premiership despite possible interest from French clubs.
"I have enjoyed my time at Franklin's Gardens and will be especially sad to leave the supporters who have been brilliant to me," said Spencer, who won 35 caps for New Zealand.
"I am still fit and strong and have certainly got a few good seasons in me yet so this is a good opportunity for me to take up a new challenge."
Saints chief executive Allan Robson confirmed Spencer's exit adding, "Carlos' departure from the club has been a mutual decision which we have both seen as an opportunity.
"Carlos has been a central figure in the first team since his arrival at Franklin's Gardens and has always been a crowd favourite. Since Jim Mallinder's arrival at the club at the beginning of last season, however, the squad has been rebuilt and reshaped.
"Together with Carlos we have taken an objective view of the situation and agree that this is in the best interest of both the club and Carlos."