Scarlets celebrated a first win over Leinster since 2018, producing a superb all-round performance in their final home match of the campaign to continue their push for a URC play-off spot.
The West Walians inflicted only a second defeat on the season on the league pacesetters and were good value for their win, scoring some scintillating tries in a 35-22 victory.
Taine Plumtree produced a totemic player-of-the-match display, scoring one try and providing the assist for another for Gareth Davies, who became the leading Welsh try-scorer in URC history.
Tom Rogers and Joe Roberts were also on the scoresheet to the delight of a pumped-up home crowd.
The win has lifted the Scarlets into seventh position in the league table ahead of back-to-back games in South Africa to finish the regular season.
The home side stormed ahead after just eight minutes. A sublime catch and off-load from prop Henry Thomas put Plumtree racing upfield and his inside ball found scrum-half Davies haring up on his inside for the score – his 55th in the Championship to go one above team-mate and good friend Steff Evans.
Costelow converted then added a penalty after Leinster had infringed at a breakdown.
Opposite number Ross Byrne responded with three points for the visitors, but Scarlets extended their advantage with a brilliantly-worked score on 21 minutes.
A strong carry by prop Alec Hepburn put the home side on the front foot, then a deft cross-kick from Costelow bounced up for Rogers, who gathered and touched down wide out.
However, straight from the kick-off Leinster pounced when centre Jamie Osborne scooped up a Costelow charged-down clearance. Byrne added the extras to make it 15-10.
The see-sawing nature of the contest continued and on the half hour mark, Scarlets were celebrating their third try of the afternoon, courtesy of Plumtree, who intercepted on halfway and raced jubilantly to the line for a converted score.
But back came the Irish side with replacement Diarmuid Mangan touching down from close range after a slick Leinster attack.
A breathless first half ended with Scarlets on the attack, but Leinster skipper Ronan Kelleher produced a key turnover after the dangerous Roberts had threatened.
The Welsh side, though, picked up where they left off and notched up their fourth try within two minutes of the restart.
A move right off the training ground saw Blair Murray veer outside the defence and he found Roberts tracking inside for a free run to the line. Costelow added the extras.
However, Scarlets failed to deal with the restart and Leinster found a way over through No. 8 James Culhane to make it a one-score game again.
Replacement Ioan Lloyd pushed the score out to 32-22 on 54 minutes, then Plumtree came agonisingly close to his second after a clever kick from Roberts.
The final quarter was a lot more cagey with Davies’s astute kicking ensuring Leinster were pinged back in their own territory, allowing Scarlets to nudge further ahead through a late Lloyd penalty and get to the finish line without any finger-gnawing late moments.
Scorers
Scarlets – tries: G. Davies, T. Rogers, T. Plumtree, J. Roberts. Cons: S. Costelow (3). Pens: Costelow, I. Lloyd.
Leinster – tries: J. Osborne, D. Mangan, J. Culhane. Cons: R. Byrne (2). Pen: Byrne.
Attendance: 7,381