Robbie Henshaw will join Leinster on a three-year deal at the end of this season.
The Ireland centre will play out the season at Connacht before joining Leinster in a deal which will see Henshaw sign until after the 2019 Rugby World Cup.
Speaking of his move from Connacht to Leinster, Henshaw said: "I would like to thank Connacht Rugby, 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 is delighted with his latest acquisition and is looking forward to bringing the up-and-coming centre to Dublin.
Cullen said: "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 RBS 6 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."
Leinster have also added former Munster lock Ian Nagle to their ranks, who will also join for the start of the 2016/17 season.
Nagle took time off for education purposes in 2014 and is currently playing with London Irish.
Cullen added: "It is well documented at this stage that Ian took a sabbatical from rugby at the end of the 2013/14 season to pursue educational opportunities most recently at Cambridge University. He has however made a return to rugby with London Irish and was involved last weekend in their win against Worcester.
"He is also relatively young at 27 and I think he will add even more experience and competition to an already competitive second row roster."
Cullen also confirmed that 17 players have signed new IRFU contracts with the province.