The 29 year-old will arrive at Hove for the 2017 season after first completing the South African domestic season this winter with Cape Cobras.
Van Zyl, born in Cape Town, has played 12 Test matches for his country and made an unbeaten century on debut in 2014, as South Africa beat the West Indies by an innings and 220 runs at Centurion.
The left-hander has played 131 first-class matches, scoring almost 8,000 runs in the process, including 18 centuries and averaging 42.78. His List A record is also impressive, having made over 3,000 runs in his 103 matches.
His medium-pace has also yielded over 50 wickets in first-class cricket, as well as a best haul of 5 for 32.
Van Zyl said of his move to Hove, “I am very excited to be joining such a prestigious club as Sussex and can't wait to be contributing on and off the field, especially with the youngsters coming through.
“I look forward to scoring plenty of runs and taking plenty of wickets in the new county season and aiming to help Sussex towards silverware next season.
“I would also like to thank all the coaching staff and board members for the opportunity for the next three years.”
Sussex’s Head Coach Mark Davis said, “We are delighted to have Stiaan joining us at Sussex Cricket. He is an established first-class cricketer of immense talent and has also represented South Africa in 12 Test matches.
“Stiaan brings a wealth of experience to the team, which is a crucial ingredient to our young developing squad. His medium pace bowling should also not be underestimated.
“To have a top order batsman who is able to bowl seam is a very important element in balancing our team for next season. He is a player who plays all formats of the game, which is another crucial part of our strategy moving forward.
“To have a player of this calibre choosing to play his cricket in England, and committing himself to Sussex, will be a huge boost to the club.”
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