Scarlets took another huge step towards the Guinness PRO12 play-offs, beating their arch-rivals Ospreys in Swansea for the first time since 2007.
A dominant second-half display saw Wayne Pivac's side earn west Wales bragging rights, with replacement fly-half Aled Thomas kicking four penalties in the final 17 minutes to seal victory.
The 25-16 win keeps Scarlets in third spot in the table and also means their opponents are all but certain to be playing their European rugby in the second tier Challenge Cup next season.
Scarlets took an early lead through fly-half Dan Jones but with Ospreys enjoying the bulk of possession and territory the home side responded with the game's first try on 16 minutes.
Fly-half Sam Davies - preferred to Wales first choice Dan Biggar - and former Scarlets full-back Dan Evans were the architects, cutting open the visitors' defence before centre Josh Matavesi took the try-scoring pass to coast over.
Davies landed the simple conversion then added a penalty as Ospreys looked to take a grip on proceedings.
But Scarlets, with their scrum on top, started to get a foothold in the contest and were level on 28 minutes thanks to a try from their Canadian speedster DTH van der Merwe. Wales scrum-half Gareth Davies made the incisive blindside break with number eight Morgan Allen feeding van der Merwe, who plunged over in the corner for his seventh league try of the campaign.
Davies landed a penalty to restore Ospreys' advantage, but Scarlets were level 13-13 at the break with another success from Jones.
It needed some determined home defence to deny Scarlets at the start of the second half, then Ospreys were denied what seemed a certain score when home centre Hadleigh Parkes intercepted on his own line.
That proved a turning point in the match as Scarlets took control in the final quarter.
Thomas came on for Jones and missed his first attempt at goal, but made no mistake after that as indiscipline cost the Ospreys dear.
Davies pulled a penalty back for Ospreys and it was 19-16 going into the final five minutes. But a rash high challenge by Sam Underhill gifted Scarlets another three points before Thomas landed the final blow with the final kick of the match.