Messam joins a growing list of high-profile players to depart the Waikato, with Aaron Cruden, Tawera Kerr-Barlow, Hika Elliot and James Lowe all leaving the club.
Messam is the former captain of the two-time Super Rugby champion franchise the Chiefs and is the clubs highest-capped player notching up 161 games.
Last month French rugby newspaper Midi Olympique reported that Messam was being chased by another French club, Lyon.
However, the southern French club Toulon got Messam's signature and he will join fellow Kiwis Malakai Fekitoa, Ma'a Nonu and Alby Matthewson.