The Scarlets secured a home quarter-final in the Guinness Pr014 play-offs after a 33-8 win against the Dragons at Principality Stadium.
Tries by hooker Ryan Elias, a penalty try, flanker Tadhg Beirne (2) and wing Steff Evans secured second spot in Conference A behind Leinster.
The defending champions will now host the Cheetahs in Llanelli after the South Africans secured third place in Conference A with victory against the Southern Kings.
It was a defeat to bring an end to a disappointing season for the Dragons, who won just twice with the Southern Kings the only team to have a worse record.
They battled hard to trail just 7-3 at the break only to run out of steam after the interval and the Scarlets got the job done before an hour was played in the first part of the Judgement Day double-header in Cardiff.
The Scarlets were swiftly into a 7-0 lead when they kicked penalties to the left corner for a driving lineout try by hooker Elias that was converted by Wales and Lions full-back Leigh Halfpenny.
They continued to dominate but could not turn pressure into points and instead it was the Dragons who struck next with fly-half Arwel Robson reducing the deficit with a penalty after 23 minutes.
The Scarlets felt they had got their second try through Halfpenny only for the try to be ruled out by the TMO because of a knock-on in the build-up by scrum-half Gareth Davies and it stayed 7-3 after a scrappy first half that would have frustrated the champions and pleased the Dragons.
However, they turned possession into points after the break on the back of a strong scrum.
A series of penalties were awarded five metres out, leading to a yellow card for replacement Dragons tighthead Chris Coleman, before referee Lloyd Linton marched under the posts for a penalty try.
It was soon 19-3 when flanker Beirne, who is moving to Munster next season to chase an Ireland spot, was put over down the left by centre Scott Williams.
And there was relief in west Wales when the Scarlets had their crucial fourth try after 56 minutes after more strong carrying by Beirne, Halfpenny adding the simple conversion by the posts.
At 26-3 the game was done and dusted but the Dragons showed spirit to respond after 63 minutes with a lovely score by wing Jared Rosser, who finished a flowing move with a diving finish down the left.
But the Scarlets had the final say when Wales wing Evans cut a nice line off his wing to powerfully finish for a try that Halfpenny converted.