Van Velze, a former IRB Junior Player of the Year nominee, came through the academy system at Loftus Versfeld and made his Super Rugby debut in 2010.
The 24-year-old number eight is now looking forward to beginning a new adventure at Northampton.
"I'm lucky enough to have got the opportunity to come here and take my career to the next level," he told SaintsTV.
"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."
Saints boss Jim Mallinder is delighted to have agreed a deal with Van Velze, who is set to fill the void left by Ulster-bound Roger Wilson.
"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."
Gilbert has released a new line of rugby cleats. The Gilbert Virtuo 8S is part of the exciting new product. Check it out.
The entire All Blacks apparel line has been updated for 2013/14. Check out the New Zealand All Blacks polo.
The Nike Tiempo is a solid rugby cleat and one of few styles still made from full-grain natural leather.
The Gilbert Blitz 8S rugby cleat is a great cleat at a great price of $69.99. Get a new pair of cleats today.
A cool looking all black rugby cleat with the high performance adidas is known for. Get in the Gear!
Wear the crest of the British and Irish Lions on your t-shirt. A great look for the summer.
The All Blacks Performance t-shirt is black with hints of blue from the training jersey. Very Cool.
The New Zealand All Blacks training jersey for 2013/14. Get in the Gear!
The USA Rugby Pro Alternate rugby jersey is perfect for any fan of the Eagles. Get yours to wear during the summer Test matches.
The NEW All Blacks 2013/14 jersey has arrived at World Rugby Shop. Dare to wear the colors of the All Blacks.