Ireland centre Robbie Henshaw is to join Leinster on a three-year deal which will run from July 2016 until after the 2019 World Cup.
The 22-year-old will see out the remainder of his contract with Connacht who are in contention for Pro12 and European Challenge Cup honours.
He looks set to fill the void left by the impending departures of Ian Madigan and Ben Te'o.
Henshaw, a regular in Joe Schmidt's Ireland squad, has won 17 caps.
The Athlone man has formed an effective centre partnership with Jared Payne for Ireland, the pair teaming up in midfield for the eighth time in Saturday's 10-9 defeat by France in Paris.
"I would like to thank Connacht Rugby, the IRFU and Leinster for allowing me to consider my future," said Henshaw.
"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. He 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."