Tries from Tommy Bowe and Lee Byrne helped the Ospreys to victory in the first ever Magners League Grand Final at the RDS.
The win, the Ospreys first ever success at the ground, ensured that the Ospreys became the first side to claim the Celtic crown on three occasions.
The 17-12 scoreline was Leinster's first home defeat in the Magners League since September 2008.
With a red-hot atmosphere, the tension boiled over early on with Alun-Wyn Jones and CJ Van Der Linde engaging in an off the ball fracas as early as the second minute. With less than 120 seconds on the clock, Dan Biggar was touching the ball down behind the posts, but referee Chris White had already blown for a clear knock-on by Marty Holah as he looked to gather a Bowe pass during the build-up.
It was a fast and frenetic opening with both sides looking to move the ball at every opportunity, only for a combination of fierce tackling and poor handling to bring a number of promising moves to a premature end.
Biggar had the opportunity to put his side ahead in the 15th minute after Shane Jennings was penalised for coming in from the side, but the fly-half's attempt from just to the left of the posts on the 22 went narrowly wide of the right upright.
The Ospreys fans were cheering five minutes later though, when Irish hero Bowe went over for the first try of the game, and his 15th of the season, taking advantage of some great play by Andrew Bishop who ghosted past the ineffective challenge of Gordon D'Arcy, Biggar's conversion making it 7-0 to the region at the end of the first quarter.
Jonathan Sexton got the hosts on to the scoreboard just a couple of minutes later with a long range penalty following a Byrne high tackle on his opposite number Rob Kearney.
The Ospreys should have extended their lead as the half hour mark approached, an excellent move seeing them recycle the ball through the phases to work a three man overlap on the left wing, only for Alun-Wyn Jones to delay his pass for too long, allowing the Leinster defence to regather, crowding out Jerry Collins.
The RDS crowd were stunned into silence five minutes later when the Ospreys extended their lead with an excellently crafted try, Biggar and Bowe exchanging passes in midfield before a perfectly weighted pass from the fly-half over the head of Bishop allowed James Hook to gather and feed Byrne who had a clear run in to score, Biggar adding the extras.
As referee Chris White blew for half-time, it was the sound of 'Hymns and Arias' that was hanging in the Dublin evening air as the Ospreys fans cheered their team back to the dressing room after an outstanding first 40 minutes.
HALF TIME: LEINSTER 3 OSPREYS 14
Sexton kicked the first points of the second half in the 45th minute, after Alun Wyn Jones entered a ruck from the side in a central position on the 22.
Biggar took the Ospreys lead back up to 11 just three minutes later with a simple penalty after Leinster were caught offside.
With the home support turning up the volume, Leinster looked to apply more pressure on the Ospreys, but with Bishop leading the way with some inspirational defensive work, the hosts were finding it difficult to find a way through.
It was tense, nervy stuff and the Ospreys were grateful to Byrne for some great covering work under the posts to snuff out the threat of Isa Nacewa as he chased a Sexton chip through.
A Leinster chargedown from an attempted Biggar clearance led to an intense period of pressure for the home side as they moved the ball left and right, hammering on the tryline. The Ospreys defence kept them out, but ultimately at the expense of a penalty, Sexton slotting it over just after the hour to make the score 17-9.
Leinster were continuing to chip away at the Ospreys, and another Sexton penalty after a high tackle on D'arcy brought them to within five points with ten minutes remaining.
The Leinster pressure continued as they looked for the crucial score that would clinch the Magners League title. Sexton spurned the opportunity to reduce the deficit to two points with just three minutes remaining and the Ospreys were able to hold on for a historic victory, sparking joyous celebrations at the final whistle