Tonga'uiha, who has made 43 appearances in ERC's tournaments scoring seven tries, will be joining Top 14 outfit Racing Metro, and said that the move to Paris was not an easy decision to make.
"My seven years at Northampton have been very happy ones," he said. "I have been with the club through relegation to promotion to the Heineken Cup final and it is a journey that has made Saints a part of me.
"Northampton is a fantastic club, with excellent coaches, management, staff and facilities. I am surrounded by a great bunch of talented lads who I believe with the addition of incoming players (not to mention the huge talent within the Academy) will go on to win plenty of silverware."
Director of rugby Jim Mallinder said that Franklin's Gardens will be a different place without Tonga'uiha in the Saints squad. "Soane has been a big part of the squad throughout my time at the Saints," he said. "He has shown consistency and durability in one of the toughest positions on the field and will go down as one of the best props to play for the club, not least with 200-plus appearances under his belt."