The announcement demonstrates yet another statement of the club's intent as they aim to build a side to compete in the upper echelons of the Aviva Premiership.
Pretorius, who was born and bred in South Africa, moved to the UK in 2006 to study Business at Hartpury College in Gloucestershire.
Having skippered Hartpury and represented England students, Andries then linked up with Gloucester's Academy, before making the move to Cardiff.
In 2011, Pretorius was awarded Players' Player of the Year and was Cardiff Blues' captain for the 2012/13 season. He won his first cap for Wales during the 2013 summer tour to Japan.
Pretorius said: "I have loved my time at Cardiff Blues and it has been an honour to play for and captain such a great team.
"I will continue to give Cardiff 100 per cent until the end of the season. My focus will then turn to Worcester.
"I have known Dean Ryan and Carl Hogg from my days at Hartpury and Gloucester. They are top quality coaches who want to take Worcester to the top of English rugby.
"Dean really impressed me with his long term vision for the club and I very much look forward to working with the coaching team to achieve their goals."
Warriors Director of Rugby Dean Ryan said: "I am delighted that Andries has chosen to join the club as we build for the future to become a competitive force in the Aviva Premiership.
"We have seen two of our top players extend their contracts last month, which demonstrates that current players and those we are seeking to bring to the club buy into our vision of building for the long-term.
"Andries has a proven track record so we look forward to welcoming him to Sixways in the summer."
Warriors Chief Executive Charles Cameron added: "Andries' signing is evidence of the development taking place at the club and shows we are moving in the right direction."