The 29-year-old has put pen to paper on a deal which will see him represent the county in all forms of the game, and he will arrive in Chelmsford next month if his visa application is successful.
"I'm thrilled to have been given the opportunity to represent Essex," Ryder told Essex's official website.
"This couldn't have come at a better time as I want to get as much cricket as possible over the New Zealand winter.
"The side looks very strong and they have got off to a good start for the season."
Ryder averages 40.93 in Test cricket, and has played 48 one-day internationals and 22 T20s for the Black Caps.