Having made his Super 14 debut with the Sharks in 2005, he went on to make his first appearance for the Springboks in 2006 against New Zealand.
Pienaar, who can play at scrum half or out-half, was a member of the victorious 2007 Rugby World Cup squad, and to date has won 36 Springbok caps and scored 85 Test points.
Speaking about Ulster's decision to recruit the World Cup-winning Springbok, Ulster Rugby Operations Director David Humphreys remarked: "Bringing Ruan to Ulster is a massive statement of our ambition to be competing with the top teams in the Magners League and Heineken Cup.
"He is one of the most naturally gifted players in world rugby and having had huge experience at international level with the Springboks, we believe Ruan will help Ulster achieve success over the next couple of seasons."
Pienaar will be joining former Springbok and Sharks players, BJ Botha and Johann Muller, in the Ulster squad next season.