Coming off some hot form in the World Cup, Styris will join fellow New Zealand skipper Stephen Fleming and batsman Hamish Marshall in the county competition.
The 31-year-old scored 499 runs at the World Cup for an average of 83.66 and took nine wickets at an average of 26.88.
New Zealand cricket manager Lindsay Crocker said Monday the spell playing county cricket would allow Styris to extend his strong form.
"Most importantly for us, it gives him the chance to continue the momentum which he built up at the World Cup," Crocker said.
Styris will return to the New Zealand team in September for the Twenty20 World Cup in South Africa and the Test series against the South Africans.