He joins the Heineken Cup champions from Perpignan where his form prompted a call up to the recent Lions tour to South Africa.
Hines will be joined by Ireland tighthead prop Mike Ross (Harlequins) and Australian back Shaun Berne (Bath) who have each agreed two-year deals with the province.
Hooker Richardt Strauss (23) has also agreed terms with Leinster and joins from South African side, the Cheetahs.
The quartet will join Ireland scrum half Eoin Reddan who had previously signed a three-year deal, transferring to Leinster from London Wasps.
Meanwhile, young scrum half Paul O'Donohoe, who made four senior appearances last season, and hooker Jason Harris-Wright have both been awarded their first professional contracts.
The talented pair, both former Ireland underage internationals, are the latest graduates from the Leinster Academy.
Leo Cullen will continue as captain after guiding the province to the Heineken Cup title last season, while there is one remaining vacancy in the playing squad.
Leinster are set to travel to Nice for a week-long training camp in the south of France, which will include a pre-season friendly against Rugby Nice Cote d'Azur on Friday, August 21 (kick-off 7pm local time).
Michael Cheika's men will continue their preparations with a game against London Irish a week later on Friday, August 28 in Donnybrook (kick-off 7pm) ahead of the start of the Magners League campaign.
LEINSTER 2009/10 Squad: Backs (15) - *Shaun Berne, Gordon D'Arcy, Girvan Dempsey, Luke Fitzgerald, Shane Horgan, Chris Keane, Rob Kearney, Simon Keogh, Fergus McFadden, Isa Nacewa, *Paul O'Donohoe, Brian O'Driscoll, Eoin O'Malley, *Eoin Reddan, Jonathan Sexton.
Forwards (19) - Leo Cullen (capt), John Fogarty, *Jason Harris-Wright, Cian Healy, Jamie Heaslip, *Nathan Hines, Trevor Hogan, Bernard Jackman, Shane Jennings, Stephen Keogh, Ronan McCormack, Kevin McLaughlin, Sean O'Brien, Malcolm O'Kelly, *Mike Ross, *Richardt Strauss, Devin Toner, CJ van der Linde, Stan Wright.
*Denotes new addition
LEINSTER 2009/10 Academy: Andrew Conway, Mark Flanagan, David Kearney, Michael Keating, Brendan Macken, Ian Madigan, Stewart Maguire, Jack McGrath, Ian McKinley, Shane Monahan, David Moore, Niall Morris, Ciaran Ruddock, Rhys Ruddock, Dominic Ryan, Paul Ryan, Tom Sexton, Eamonn Sheridan, Eoin Sheriff, Kyle Tonetti.