Western Force forward Steve Mafi is one of seven new signings announced by Castres on Tuesday ahead of next season.
Mafi spent four years with Leicester Tigers from 2010-2014 before returning to Australia to join the Force where he in his second campaign with the Perth-based franchise.
The 26-year-old has signed a two-year deal with Castres, with the option of a third.
Castres have also confirmed the recruitment of Montpellier back Robert Ebersohn, Albi centre Afusipa Taumoepeau, Chiefs back-row Ma'ala Vaipulu and props Daniel Kotze (Clermont), Damien Tussac (Montauban) and Tudor Stroe (Tarbes).
Those signings join other new faces in Oyonnax hooker Jody Jenneker and Clermont lock Loïc Jacquet.