Both sides played some excellent rugby throughout a fantastic match which saw Dragons prop Phil Price, on his 50th game for the side, and full-back Dan Evans cross for the home region while centre Scott Williams got a cracker in reply.
Ex-Wales wing Tom Prydie booted two long-range penalties, with fly-half Jason Tovey adding both home conversions and a penalty.
Wales outside-half Rhys Priestland booted three Scarlets penalties and converted Williams' try but, despite some intense pressure in the final throes of the game when a converted try would have drawn it, the Scarlets could not cross again.
Two British Lions were back in regional action on opposite sides as Dragons number eight Toby Faletau started as did Scarlets centre Jonathan Davies.
Flanker John Barclay also made his seasonal debut for the West Wales side after a shoulder injury.
With both sides having won and lost one of their opening matches so far, there was all to play for right from kick-off.
And the match did not disappoint in the first half as both sides played some excellent running rugby.
There were just two scrums in the period which played into the Dragons' hands as the Scarlets looked in mean mood during their 26-10 home victory over Benetton Treviso last Saturday.
Former Wales cap Prydie was in a dangerous mood early on as he thumped over a 45-metre penalty to get the Dragons rolling and then nearly got a try in the right corner when he sprinted 35 metres but was found to have put a foot in touch just inches short.
Faletau caught the Scarlets napping moments after Prydie's score when Priestland's attempted clearance from inside his own try area was charged down by the Lions back-row but only for the ball to fly dead.
But the Dragons would not be denied and, after heavy pressure inside the visitor's 22, the ball found its way wide on the left where scrum-half Richie Rees found Price to drive over.
Tovey converted but eventually the Scarlets got their act together and, when they moved to the home 22, the Dragons infringed at a ruck, giving Priestland an easy three points.
It was turning into a roaring game although the amount of aimless kicks down field by both sides was annoying.
Almost directly from the restart of Priestland's penalty, the Dragons had the Scarlets back on the rack as the visitors infringed inside their own 22 and never saw the other side of it until Evans had finished off a neat move to score, with Tovey converting.
It put the Dragons well ahead at 17-3 but the Scarlets, led admirably by number eight Rob McCusker, hit back.
Priestland got their second penalty before the Scarlets danced down field and pressurised the home side before the two centres combined for Davies to send Williams in for the converted touchdown.
The intensity continued after the break with Priestland bringing the Scarlets back to within a point before Tovey's penalty and Prydie's 40 metre strike put the home side seven points ahead.
The finale was huge with the Scarlets having a scrum on the home line which went on past the 80 minute mark before the fourth reset saw the Dragons win possession and boot the ball into touch to secure their derby win.