Van Velze, a Number Eight who stands at 1m 92 and weighs 107kgs, joins from the Blue Bulls, where he has come through the Academy system to be regarded as one of the country's highest rated young players.
The Bulls' website says that "they don't come tougher than this" 24-year-old, who twice captained the province's junior team to the Vodacom Cup. Van Velze also captained South Africa Schools and was a nominee for the IRB Under-19 Player of the Year in 2007 before working his way through the ranks at Loftus Versfeld to become a key member of the Bulls' Currie Cup squad and make his Super Rugby debut in 2010.
However Van Velze is excited that the next stage of his career will be spent at Franklin's Gardens.
"I'm lucky enough to have got the opportunity to come here and take my career to the next level," he told SaintsTV in an exclusive interview. "We see a lot of the games in South Africa with Saints being one of the biggest clubs, so we all know of the team and how it plays.
"I'll be filling big boots, that's for sure. But I'll be coming to a club that's very professional and with people who work hard and want to win trophies. I've been very impressed with the club; it's somewhere which can be compared to the best clubs in world rugby."
Director of rugby Jim Mallinder says that Van Velze has been a target for some time and that he is delighted that the player has put pen to paper.
"We've been following Gerrit-Jan's progress for some time and are very happy that he is coming to Franklin's Gardens in the summer," he said. "He will bring plenty of qualities to our squad - he carries the ball well, has lots of pace and shows good game understanding and leadership.
"We will need all of those things as we look to stay at the top end of the Premiership and get to the knockout stages of European competition, and believe that Gerrit-Jan can play a big role in our development as a squad."
Watch Gerrit-Jan in action here: