Pienaar has played 88 games for South Africa with his last appearance coming against Argentina in the 2015 Rugby World Cup in England.
"I'm really looking forward to arriving in Montpellier." said the experienced playmaker
"This is a new challenge for me. The Montpellier Hérault Rugby is a large club, with a team with so many world-class players. And it's also a wonderful city!
"I look forward to seeing some of the old friends I've met in South Africa, but also to meet other players, whom I know only by name. I also hope to learn French, and to benefit from all that France, and more particularly Montpellier, can offer me."