The man who has scored more than 1,000 in 111 appearances for Leinster over the past six years has singed a four year deal with the Top 14 outfit. He will leave Dublin at the end of the season and join the French club shortly after his 32nd birthday in August.
"This is one of the hardest decisions that I have ever had to make in my career. I spoke with my family about it at length, but I feel that the end of this season is the best time to make a new start," he said.
"Since joining Leinster I have seen the team grow every year, both on and off the field. I feel a strong link with Leinster.
"We have three important months left in the season and my sole intention now is to do my best to try to help to win the Heineken Cup and retain the Magners League title for the club. It has been an incredible journey and I'd like to finish it on a high."