He joins fellow All Blacks Luke McAlister, Malakai Fekitoa and Ma'a Nonu at the French Top 14 club.
It's Mathewson's latest stop in a career which has taken him from the Hurricanes to the Blues to the Western Force and domestic stints with Wellington, Hawke's Bay and Canterbury.
He had a stint in England last season with Bristol, but has most recently been plying his trade with King Country in the Heartland Championship.
The 31-year-old, who played four tests for the All Blacks, agreed a deal that includes an option for a second season in the south of France.