Viljoen, who made his Derbyshire debut in 2017, requested to have the final year of his contract terminated and leaves with immediate effect.
The 29-year-old claimed 62 first-class wickets at an average of 28. In total he played 53 matches across all forms for the Club, taking 99 wickets.
Head of Cricket, Dave Houghton, said: “Hardus wishes to look at different options to further his career and the decision was made to release him from his contract with Derbyshire.
“I would like to wish him all the best for the future.”
© Cricket World 2019