Ruan Pienaar will join Ulster at the start of September after agreeing a release from his Natal Sharks contract.
Pienaar, 26, can play both scrum-half and fly-half was part of South Africa's World Cup-winning squad in 2007.
He is currently on duty with the Springboks in the Tri-Nations but had already announced his move to the Ravenhill side in May.
His two-year deal will commence on September 1 although he will be allowed to play for South Africa if selected for their Tri-Nations finale against Australia on September 4, meaning he could miss Ulster's Magners League opener against the Ospreys.
"We're delighted to have concluded the contract negotiations around Ruan's move to Ulster," said Ulster operations director David 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.
"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, who will join fellow South Africa internationals Johann Muller and Pedrie Wannenburg in Belfast, said: "I'm very excited about the change - I have been thinking about going to the northern hemisphere for a couple of seasons now and when the opportunity to join Ulster came up I was very interested."