The powerful and experienced 27-year-old, who can also play on the wing and has been capped four times for New Zealand, will join his new club next summer.
"This versatile player has a profile that we were missing in our squad. He's going to bring lots of strength to the middle of the pitch," said Clermont's sporting director Franck Azema.
He added: "He's excellent in defence and in attack is capable of making the difference as much with his side steps as his strength."
Injury-ravaged Clermont could use Moala now as Alivereti Raka, Camille Lopez, Noa Nakaitaci and former All Black Luke McAlister are all sidelined.
Clermont have won all four European matches this season to top Pool 2 by five points from Ospreys with two matches to play.
But domestically they are ninth in the Top 14 standings after 12 matches.