The 24-year-old scrum-half has played for the Maori All Blacks and was part of their squad that toured England in 2012.
The Guinness PRO12 side have also confirmed that Hayden Triggs has extended his contract and will remain with the club for the 2016/17 season.
Commenting on the news, Leinster Rugby Head Coach Leo Cullen said: "We are very happy to have secured two key signings ahead of next season.
"Jamison, for a young player, has a wealth of experience behind him already and was an ever-present in the Blues team last season in Super Rugby before signing with the Hurricanes.
"I think he will bring great competition to an area where we are well-serviced with the likes of Nick McCarthy and Charlie Rock pushing the senior players hard already.
"Hayden has been influential since joining us earlier this season and we are probably only now seeing the best of him as he unfortunately suffered that ankle injury against Bath in the Champions Cup.
"In 14 games for us to-date we have seen the leadership that Hayden brings to this team as well as his excellent work ethic with and without the ball."
Lock Denton will join Gloucester after four seasons at Leinster, having arrived from Leeds Carnegie (now Yorkshire Carnegie) back in 2012.
So far this season, Denton has made ten appearances for Leinster, including appearing as a replacement in three Champions Cup pool stage games.
Denton said that while he's looking forward to returning to the Premiership, there is some unfinished business to take care of with Leinster first.
"I'm very excited about the prospect of returning to England and having another crack at the Premiership with a club like Gloucester, one with a great history but a bright future as well.
"I feel that my game has developed a lot while with Leinster and I know that I'll need to perform well as Gloucester already have a number of quality second rows.
"However, we've got a big semi-final against Ulster to prepare for next Friday in the RDS and my focus is completely on that as I'd love to bring my time at Leinster to a close having helped to secure the Guinness PRO12 title."
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Guinness PRO12 Leinster News
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