COREY Domachowski proved the unlikely match-winner for Cardiff as he snatched a dramatic last-gasp try in the 29-24 victory at Rodney Parade.
The Dragons thought they had ended their 14-game losing streak against their local rivals as Harrison Keddie had followed Rio Dyer over the try line to overturn the deficit late on.
But replacement prop Domachowski barged over 30 seconds from time, after two from scrum-half Tomos Williams either side of Josh Adams' try, to sneak a thrilling win and move Cardiff up to fifth.
Cardiff head coach Dai Young said: “It was a tough, physical game and this is a tough place to play so we are thrilled to go home from a real dingdong Welsh derby with five points.
“It came down to perseverance right at the end. It looked a bit of a mountain but we got a great turnover, a fantastic kick from Jarrod and then a couple of big carries got us over.”
In the other Welsh derby over the festive period, Ospreys defeated 14-man Scarlets 34-14 after Argentina flanker Tomas Lezana was shown a fifth-minute red card.
Lezana was sent off for a dangerous tackle and Ospreys took full advantage, running in tries from Alex Cuthbert, Luke Morgan, Rhys Webb, Sam Parry and Keiran Williams.
Scarlets responded with tries for Sione Kalamafoni and Johnny McNicholl but the match ended with Ospreys picking up just their second league win of the season. Glasgow Warriors put one over on their old rivals Edinburgh with a narrow 16- 10 victory at Scotstoun.
Jack Dempsey got his legs pumping to power over the try-line as Warriors established a 10-point half-time lead.
Edinburgh's secondhalf improvement was marked by Connor Boyle, holding off two players to dot down after a maul. Two second-half penalties by stand-off Tom Jordan sealed the win. Unbeaten table toppers Leinster secured a 20-19 away win over Munster in a tight game at Thomond Park. Gavin Coombe's converted try put Munster ahead 7-6 in the first half.
Tries in the second half from Scott Penny and Dan Sheehan edged Leinster ahead.
Munster were awarded a penalty try and a score from replacement Patrick Campbell set up a tense finale, but it was not enough to rescue the game for the hosts.
Ulster hung on for a dramatic 22-20 win away at Connacht and move up to third in the overall standings.
Ulster held a 19-3 lead courtesy of Tom Stewart's try double but Connacht surged back into the game with two late tries.
Jack Carty missed a conversion with the final kick of the game as the hosts fell just short of a dramatic draw.
Springbok wing Makazole Mapimpi's brace inspired the Sharks to a 37-10 win over the Lions, and the Stormers saw off the Bulls 37-27 courtesy of 15 points from the boot of deadeye kicker Manie Libbok to move up to second.
In the Italian derby, Benetton scored six tries as they thrashed winless Zebre 38-5 with Lorenzo Cannone grabbing two of them.
