GJ has been a regular in the Heineken Cup and has a reputation as one of the brightest back row prospects from South Africa.
Van Velze said: "I had a conversation with Dean Ryan, during which he explained the challenge ahead.
"It's a different challenge to the one I've had at Northampton but it's something I'm looking forward to.
"I know Nick Johnston from his time at Northampton so it is clear that players are looked after on and off the pitch and it is an environment which will allow me to grow as a player and a person.
"I've enjoyed every moment of my time at Northampton and credit must go to all the backroom staff for all they've done. I have been proud to represent the club on all levels during the last two seasons.
"I would like to thank all the players I have been privileged to play with and all the supporters who made me feel at home from day one. Franklin's Gardens will always have a special place in my heart."
Warriors Director of Rugby Dean Ryan said: "It is fantastic that GJ has decided to join us. He is an experienced Premiership player with strong leadership qualities who we see being influential on the pitch as well as joining a strong leadership group to help us build a new culture here at Worcester."
At the same time, Warriors are also delighted to announce the signings of upcoming youngsters Charlie Mulchrone, Joe Rees and Heath Stevens who will all move to Sixways this summer.
Twenty-four-year-old Mulchrone, who has already demonstrated his leadership abilities through his captaincy of Rotherham Titans this season, is predominantly a number nine, but also plays at fly-half.
Mulchrone said: "You only have to visit Sixways Stadium to see the facilities, management and quality of players to realise the environment being created at Worcester.
"Dean's vision for the club going forward is the perfect fit for my ambitions and I am certainly looking forward to being part of it.
"The opportunity was something I couldn't turn down. Rotherham have been fantastic for my development as a player and for the remaining half of the season my focus will be on giving my all for Rotherham until the last game is played."
Tighthead Rees, who has made 30 appearances for the Ospreys and is a former Wales representative at Under 18 and Under 20, said: "I am looking forward to starting the next part of my career this summer and am excited by the challenge ahead.
"Dean Ryan has a clear idea of where he wants Warriors to go over the next few years and I am delighted that I will be part of that."
Twenty-one-year-old Stevens has appeared in all 13 Championship games for Plymouth Albion this season and is a powerful centre.
He said: "I am over the moon to secure a move to Worcester Warriors and I can't wait to join up with my new teammates in the summer.
"I have enjoyed my time at Plymouth and will continue to work hard until my contract comes to an end."
Chief Executive Charles Cameron added: "The announcement of these four signings is another key development as we look to build for the future and we look forward to welcoming them to the club."