The stalwart has earned 95 caps along with 13 tries for the Cape Town franchise and the move to the Champions Cup winners will be the first time he represents another club.
After alternating between inside and outside centre for a long time, du Plessis gradually settled into No.12.
He joins Bordeaux to strengthen the centre position and compensate for the long absence of the injured Rohan Janse van Rensburg.
Bordeaux announced du Plessis' signing along with England duo Tom Willis and Alex Moon, Junior Georgian prop Lasha Pkhakadze, as well as French teenagers Thomas Delpeuch and Hugo Avogadro.