A Sam Davies drop-goal with the last kick of the match denied the Scarlets victory in a rain-soaked Guinness PRO14 encounter at Rodney Parade.
The clock was in the red when Davies’s left foot secured a 22-20 win in a tense first derby of the festive season.
Tries from Gareth Davies and skipper Ken Owens had nudged the Scarlets ahead in the first half, but for the majority of a scrappy second period only a point separated the sides.
Leigh Halfpenny, who enjoyed another exemplary match with the boot, looked to have won it for the Scarlets with a penalty four minutes from time, but it was man of the match Davies who had the final word.
In a dramatic opening, the Scarlets thought they had a try on the board with just nine seconds on the clock.
Rhodri Williams spilled the kick-off, opposite number Gareth Davies hacked through and Halfpenny won the race to the ball, only for the officials to rule he had knocked the ball on.
Instead, Halfpenny had to make do with a penalty to open the scoring.
With the visitors falling foul of referee Ben Whitehouse, Sam Davies booted the Dragons into a 9-3 lead with three successful penalty kicks.
But the game swung with the sin-binning of home lock Matthew Screech on 22 minutes as the Scarlets pressed for the game’s first try.
From a scrum that followed, scrum-half Davies sniped, dummied and darted over at the posts with Halfpenny landing the conversion from straight in front.
The vocal travelling support were celebrating a second score soon after.
A brilliantly-executed line-out drive saw the Scarlets rumble forward 20 metres and it was skipper Owens, playing his first game of the campaign, who touched down.
Halfpenny added the extras to push his side 17-9 ahead, but the Dragons struck back just before the break when home captain Rhodri Williams found a gap in the defence at the posts to cross. Wyn Jones was shown yellow and Davies converted to make it a one-point game at the interval.
With conditions worsening, the second half proved a cagey affair dominated by the boot.
Johnny McNicholl went close to a third Scarlets try, but was unable to hold onto a Ryan Lamb crossfield kick.
Davies nudged the home side back in front with a penalty with seven minutes remaining before Halfpenny’s response.
Then came the final drama as Davies missed with a penalty attempt then quickly made amends by snatching victory at the death.
Dragons - try: R. Williams. Con: S. Davies. Pens: Davies (4); D-g: Davies
Scarlets - tries: G. Davies, K. Owens. Cons: L. Halfpenny (2). Pens: Halfpenny (2)