The IRFU have announced that Robbie Henshaw will see out the remainder of his contract with Connacht before joining Leinster on a three-year deal which will run from July 2016 until after the 2019 Rugby World Cup.
Henshaw joined the Connacht Academy in 2012, and made his senior Connacht debut against Cardiff Blues in his first year.
His first senior cap came during the summer tour of 2013 and he has been a regular in Joe Schmidt's squad winning 17 caps to date.
IRFU Performance Director David Nucifora said, "I am disappointed for Connacht that Robbie has decided to leave but pleased for Irish rugby that he has committed to a further three years. I know that the decision Robbie has taken to move from Connacht is not one he has taken lightly."
Robbie Henshaw said, "I would like to thank Connacht, the IRFU and Leinster for allowing me time to consider my future.
"This has been a long and difficult process and while I am sorry to be leaving Connacht I look forward to the next chapter of my rugby career at Leinster.
"The support I have enjoyed from the fans in the Sportsground and throughout the province is something that I really value and I want to thank them for four great years."
Leinster head coach Leo Cullen commented, "We are delighted to have secured Robbie's signature for the next three seasons. Robbie is a huge talent and we have all seen that over the last few weeks and months with Ireland in the Six Nations and during the World Cup.
"We look forward to welcoming him into Leinster when this season is over and I am sure that our supporters would echo that view."