Spies won 53 caps for South Africa between 2006 and 2013 but missed the 2007 World Cup-winning squad because he was found to have blood clots in his lungs.
Montpellier released Spies, who was beset by injuries throughout his career, after one season with the French club following the conclusion of their Top 14 campaign in May.
Former Springboks coach Jake White had been in charge of Montpellier since 2014 but Kiwi coach Vern Cotter takes the reins for next season.
