Holwell, 29, signs ahead of this week's Heineken Cup deadline and is the highest points scorer in Hurricane's Super 12 history having accumulated 613 points in the Super 12. He signed off with a 17-point tally for Wellington in their 73-28 victory over Taranaki last weekend in the NPC.
Leinster Chief Executive Michael Dawson said: I am delighted to welcome David to Leinster. He will bring a wealth of experience and enhance levels of competition within the squad ahead of our Celtic League and Heineken Cup campaigns.
Holwell has been given an early release from his Wellington RFU and NZRU contract in order to enable him to take up the offer.
The much-respected servant of Wellington has been injured for significant periods of the past two seasons, and others such as Riki Flutey have stepped up to the plate in his absence.
Holwell is a noted kicker and was very impressive for Wellington when Ireland last toured New Zealand. He has made 58 appearances for Wellington since making his provincial debut in 1995 and has played Super 12 with the Hurricanes since 1998.