Lawes has spent his entire 17 seasons in senior rugby with his hometown side and had previously said he wanted to remain a one-club man.
But, having called time on his 105-cap England career after last year's World Cup in France, a lucrative move to the French second-tier club has proved too strong to resist for the back-five forward.
"I want to make it clear that I really would have liked to end my career as a one-club man, and Saints did absolutely everything they possibly could to make that happen - our conversations were all very positive," Lawes said in a club statement.
"But, first and foremost, I have to make sure that my family and I are in the best position possible for my retirement, which will be in the next couple of years."
Lawes, who has also played five Tests for the British and Irish Lions, added: "This is likely to be the last contract I'll ever sign, and the offer I have received to play overseas will be transformational for my family, so there was no way I could turn it down and I took the decision to move away from Northampton."
Lawes, who has played 274 matches for Northampton, is following a number of other England internationals in making the switch to France, including fellow former England skipper Owen Farrell, who is joining Top 14 side Racing 92 next season.
