Pienaar, 26, will join the Magners League outfit in September after featuring regularly in South AfricaÃ¢â‚¬â„¢s Tri-Nations campaign.
Former Ulster and Ireland fly-half David Humphreys, now the Operations Director at the club, was excited by the prospect of Pienaar moving to Northern Ireland.
“We’re delighted to have concluded the contract negotiations around Ruan’s move to Ulster,” said Humphreys.
“I would like to thank the Natal Sharks for their help in the matter and for agreeing to Ruan’s early release from his Sharks contract.”
He added: “We look forward to welcoming Ruan to Belfast in September, he is without doubt one of the biggest signings made in Ulster Rugby’s history and I’m confident that he will be a key asset to us both on and off the pitch.”
Pienaar has made 34 appearances for the Springboks since making his debut in 2006, and has scored over 80 points.
MORE: The latest football news
MORE: The latest tennis news
BIOGRAPHY: Daniel Sturridge
BIOGRAPHY: Kepa Arrizabalaga