Their long wait for that elusive fifth star goes on, as La Rochelle etched their name on the Heineken Champions Cup for the first time.
A coaching masterclass led by Ronan O'Gara saw the Munster legend join his opposite number Leo Cullen, as the only men to have won this great tournament as a player and as a head coach.
One of the great European finals was in the balance right until the death, when Arthur Retiere came off the bench to snatch the decisive try.
It was no more than La Rochelle deserved, as they scored three tries to Leinster's none, mixing brutal physicality with more subtle brilliance. Any team that keeps this Leinster side tryless are deserving champions.
The Stade Velodrome rocked as chants of 'Ici, ici, c'est La Rochelle' rang around the crowd 59,682 at full-time on a glorious day in Marseille.
Leinster went back to the drawing board after they were beaten by O'Gara's men in last year's semi-final, but they were never allowed to get into their groove here, as La Rochelle won the breakdown battle and stifled their attack.
Leinster will have huge regrets, not least when Johnny Sexton's error allowed La Rochelle to mount their late comeback.
Just as it looked as though the Leinster defence would cling on, La Rochelle scored a converted try with the last play of the game. In truth, there was a sense of inevitability about the closing stages, as the blue jerseys were struggling to cope with the power game.
Thomas Lavault's brainless second-half yellow card almost cost his side, but to their credit, La Rochelle dug deep to get the job done.
Sexton had already been roundly booed when his name was read out before kick-off, and the jeers continued as he lined up a fourth minute shot at goal. But from 38-metres out, he held his nerve to bisect the posts.
La Rochelle's discipline was really hampering their early progress, and as their penalty count rose, so too did the frustration levels of the French side, who had Skelton given a stern talking to by Barnes.
A fifth penalty concession inside the opening seven minutes allowed Sexton kick Leinster into a 6-0 lead, as the baying crowd became even more incensed.
But the La Rochelle supporters were soon on their feet applauding a stunning try from what was their side's first real attack.
As they did throughout, Dillyn Leyds and Raymond Rhule linked brilliantly, with the latter taking the offload and scorching beyond Hugo Keenan's attempted tackle before dotting down. Ihaia West nailed the tricky conversion as La Rochelle went in front for the first time on 10 minutes.
Suddenly, it was all La Rochelle, as Leinster were clinging on for dear life. Having already lost a lineout, Leinster then conceded a penalty at the first scrum, whilst also losing Rónan Kelleher to injury.
Leyds thought he was in for a second try, only for Leinster to be bailed out for an earlier infringement. The Leinster lineout was creaking, but La Rochelle's continued to cough up cheap penalties, the latest of which allowed Sexton kick his side 9-7 in front.
But Leinster were rocking and had Brice Dulin not knocked on out wide, they would have been in even more trouble. Back we went for a penalty, and La Rochelle opted for a scrum.
The initial scrum was reset and as La Rochelle looked to put the squeeze on, the Leinster front-row eked out a massive penalty against the head. Again, the crowd bayed for blood.
A minute later, it was Leinster who had a five-metre scrum after Dulin hesitated in his own in-goal area, with Jamison Gibson-Park and Jimmy O'Brien doing well to force the error.
A rehearsed set-piece move didn't come off but La Rochelle finished the half as they started it by conceding a penalty, which Sexton slotted.
Leading 12-7 at the break, Leinster needed to steady the ship after the restart but they gave away a penalty, to allow West cut the cap.
Sexton, however, quickly cancelled that out, but having been pounding away at the white wall camped on their own line, they might have come away with more.
As it was, La Rochelle's discipline issues were still proving costly and Sexton further punished them with a sixth successful shot at goal after 53 minutes.
But Sexton then endured a moment to forget when he opted to run the ball from deep rather than find the safety of touch. La Rochelle pounced and forced the penalty, from which the lineout maul obliterated the Leinster pack.
Pierre Bourgarit got over and when West added the extras, we were back to a one-point game (18-17) with 18 minutes left on the clock. That mistake proved to be Sexton's last act, as Ross Byrne came on in his place.
Just as the momentum swung back in the French side's favour, Lavault lost his head when he stupidly tripped Gibson-Park for a yellow card and three points, which followed courtesy of Byrne.
Despite their numerical disadvantage, O'Gara's men cranked up the pressure again. Having declined the three points on offer, they opted for the scrum. Leinster somehow avoided a yellow card but it mattered little as one long reach from Retiere clinched the title.
Scorers – Leinster Sexton 6 pens, R Byrne pen. La Rochelle: Rhule, Bourgarit, Retiere try each, West pen, West 3 cons.
Leinster – H Keenan; J O'Brien, G Ringrose, R Henshaw, J Lowe; J Sexton (capt) (R Byrne 62), J Gibson-Park (L McGrath 75); A Porter (C Healy 63), R Kelleher (D Sheehan 15), T Furlong (M Ala'alatoa 63); R Molony (J McCarthy 77), J Ryan; C Doris (R Ruddock 67), J van der Flier, J Conan.
La Rochelle – B Dulin; D Leyds (J Favre 67), J Sinzelle, J Danty (L Botia 69), R Rhule; I West, T Berjon (A Retiere 65); D Priso (R Wardi, 54), P Bourgarit (F Bosch 67), A Atonio (J Sclavi 62); T Lavault (R Sazy 75), W Skelton; W Liebenberg (Bourdeau 60), M Heddad (R Bourdeau 30-38), G Alldritt (capt).
REF: W Barnes (England)