Kaino made the announcement in Auckland today saying the deal was worth significantly more money than what was offered by the New Zealand Rugby Union.
Kaino comes off contract with the NZRU this year. He is currently injured and will miss the remainder of the Super Rugby season with a shoulder injury.
Kaino said he only made up his mind to leave New Zealand recently and that it was a very "tough decision". He also said calling All Black coach Steve Hansen and inform him of the move was hard to do.
"He was disappointed. That was probably the hardest phone call I had to make. Along with ringing Richie (McCaw). But he wished me the best," Kaino said.
Kaino also said the All Black skipper wished him well in Japan.
Kaino, who turns 29 next month, said he wants to back in New Zealand in time to play in the next Rugby World Cup in 2015.
"This is only for now. That's what I wanted to do. I wanted to get some experience overseas and in a couple of years time probably come back and challenge for that jersey again.
Kaino has been capped 48 times for the All Blacks and 85 for the Blues.
He has emerged from a talented but inconsistent international into one of the All Blacks' great No 6s.
He played all but 90 seconds of last year's triumphant World Cup campaign, picking up the New Zealand Player of the Year award and finishing runner-up to Thierry Dusautoir as IRB player of the year.
The 28-year-old is one of the best-paid players in New Zealand, with only Richie McCaw, Dan Carter and Sonny Bill Williams thought to be on a higher retainer from the national body.
- Kris Shannon
