Adams, 29, a lively pace bowler and aggressive batsman, has been playing club cricket in England for Lancashire League side Colne.
He could make his Essex debut on Sunday in the totesport league at Old Trafford.
Essex chief executive David East said: "We are delighted that we've been able to sign Andre. His international experience will be very valuable in all forms of cricket and we are sure he will make a major contribution during his time with the club."
Adams has played 31 One-day Internationals and one Test for New Zealand. In Limited Overs Internationals he has taken 44 wickets at 28.11 and scored 338 runs at 19.88.