Racing 92 fly-half Remi Tales missed a late drop-goal as Leinster clung on to claim a 15-12 victory in the Champions Cup final in Bilbao.
Having trailed since the fourth minute, Leinster only took the lead in the 79th as Isa Nacewa struck the second of his two penalties -- both of which came in the final 10 minutes.
Johnny Sexton had scored three penalties of his own before that but his side looked to be heading to defeat in a try-less yet absorbing encounter at the San Mames Stadium.
Teddy Iribaren, deputising for the injured Maxime Machenaud, had kept Racing's noses in front for much of the match and ended with 12 points. But it wasn't to be enough.
Leinster have never lost a European final and they managed to maintain that proud record with Nacewa's late contribution ensuring that they ended their Champions Cup campaign unbeaten.
The Irish province have now won four European Cups -- equalling Toulouse' record -- while Leo Cullen becomes the first man to win the tournament as both a player and coach. Sexton, Nacewa, Cian Healy and Devin Toner have all been part of each Leinster success.
Tales had been a late call-up to Racing's matchday 23 when Dan Carter was forced to withdraw with a hamstring injury, and he wasn't left on the bench for long.
Less than three minutes were on the San Mames Stadium clock when Racing's starting No. 10 Pat Lambie went down injured following an encouraging break.
The Springbok would not be able to return, meaning Racing had effectively lost two fly-halves before a point had been scored and thrusting Tales into the action.
Their anguish was soothed slightly due to a penalty resulting from a Garry Ringrose high tackle in the ensuing passage of play, and Iribaren sent it spiraling through the posts to give his side the lead.
Iribaren was brilliant in the opening half as he showed no signs of any nerves -- putting in a couple of booming early clearance kicks and keeping the scoreboard ticking.
Sexton levelled the scores with a penalty of his own in the opening quarter and the two kickers would exchange further three-pointers to leave the scores level at 6-6 at the break.
In between times an absorbing and brutal, if at times ugly, first half had played out in Bilbao as Leinster counteracted Iribaren's sniping with a kicking game that targeted their opponents' back three's lack of height.
The second half played out to a similar pattern, with Iribaren and Sexton exchanging penalties at the atmosphere grew increasingly tense in northern Spain.
Racing took what appeared to be a decisive lead with less than 10 minutes remaining when Iribaren slotted a nerveless three points.
Leinster, though, found some reserves of energy and got their reward for a period of possession on the opposition 22 as Racing were penalised at the breakdown.
Sexton had been dazed in the passage of play that preceded it, meaning kicking responsibility was passed to captain Nacewa who made no mistake.
There was a similar outcome less than five minutes later as the New Zealand-born wing punished the French side for getting caught offside after Garry Ringrose had taken his side into the 22.
The successful kick set up an anxious closing minute, but despite working their way into drop-goal range, Tales hooked his effort wide of the left-hand upright and Leinster were champions again.