Tulou, 28, had been at Montpellier since 2011 but has now signed with the 2013 French champions and 2014 runners-up until the end of the 2016/17 season.
Fellow backrower Alexandre Bias will also return to Castres, who finished one spot above relegation this season and who will next season be coached by Oyonnax's Christophe Urios.
In the French capital, promising 22-year-old centre Jonathan Danty signed his first professional contract with Stade Francais.
The deal will see the powerfully-built back linked with Stade, who play Parisian rivals Racing-Metro later Friday in a Top14 quarter-final play-off, until 2018.