Hines had spent four seasons with French side Perpignan having previously enjoyed a six-year spell with Edinburgh.
The 32-year-old Australian born former rugby league player has agreed a two-year deal with the Heineken Cup winners and will therefore link up with fellow Lions Luke Fitzgerald, Jamie Heaslip, Rob Kearney and Brian O'Driscoll.
"I'm really excited about the opportunity to join a Leinster side that are coming off the back of a successful past few seasons," said Hines, who currently has 58 Scotland caps to his name.
"Leinster were always a good side but the Magners League and Heineken Cup successes in recent years have bred a new culture and I'm determined to work hard and try and become a part of this successful team.
"There's a real determination in the squad to push on in both competitions and I'm looking forward to improving my game.
"I see this new challenge as a dream move and my family are looking forward to starting our new life in Ireland."
Hines will also be joined at Leinster by Ireland prop Mike Ross, Australia A playmaker Shaun Berne and South African hooker Richardt Strauss.
Ross joins from Harlequins, while Berne moves to Dublin from Bath and Strauss arrives from the Cheetahs.