The 23-year-old, who can play as either a winger, centre or full-back, will join the Irish province in July 2016 on a two-year contract.
The All Blacks' policy of not selecting players based overseas means that Piutau is essentially putting his international career on hold by making the switch, but he insists that he is "looking forward" to the challenge.
"I am looking forward to moving to Belfast, experiencing a new culture and playing in the Northern Hemisphere," Piutau is quoted as saying by BBC Sport.
"It was a difficult decision to leave the Blues and New Zealand Rugby as I have really enjoyed my time with them so far. However when I spoke with Ulster I was impressed with what they had to say and I am excited about their future plans.
"I can't wait to get stuck in with my new teammates in July 2016."
Piutau, who has won 14 caps for New Zealand and is regarded as one of the country's brightest young talents, could have also put his place at this summer's World Cup in jeopardy by agreeing to the move.