The 31-year-old loosehead prop will be leaving Franklin's Gardens after seven years' service and over 200 appearances in the black, green and gold. His career at the Saints includes a man-of-the-match performance in the successful 2010 LV= Cup Final, an Amlin Challenge Cup winners' medal, an appearance in the 2011 Heineken Cup Final and selection in several Aviva Premiership Rugby Dream Teams.
Tonga'uiha will be playing in the Top 14 next season, and says that the move to France 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.
"I would like to take this opportunity to thank all those who have supported me during my seven years here. In particular, my family and I have been overwhelmed by the generosity, kindness and support we have been shown by the people of Northampton during my Testimonial year.
"This generosity has enabled me to donate £20,000 to charitable causes as well as to ship a 40ft container filled with donations to my old school, Vaini Primary in Tonga. This would not have been possible without the help of my dedicated testimonial committee led by Jon Evans and Darren Briers, and also the faithful fans of Northampton Saints.
"Finally, this is not a goodbye for the time being as there is still plenty to play for. Saints are back in the top four and this season is far from over. I hope to leave this club in a high and I still believe we have a team that can go all the way."
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.
"Soane is moving on as an individual and we will move on as a club, and now that the speculation regarding his future is over we can all put our focus on ending the season in style and getting into the Aviva Premiership play-offs.