Scarlets 16 Ospreys 23
Ryan Jones and Hanno Dirksen's late tries saw the Ospreys shock the Scarlets and take the derby honours with a 23-16 triumph.
In a tense and scrappy opening period, largely dominated by the kicking prowess of the respective fly-halves, Dan Biggar slotted three penalties to Rhys Priestland's two.
And after Priestland had levelled the scores with his third three-pointer, Jones and Dirksen touched down late on to give the visitors their first win of the campaign, despite a late try from George North.
The two sides have endured contrasting fortunes so far this season. While the Llanelli-based outfit have emerged victorious in their first three encounters, the Ospreys have recorded the opposite - defeat in every game.
It was the hosts who started the brighter and looked to take advantage of a potential lack of confidence in the visitors' ranks.
And they opened the scoring with a Priestland penalty from a breakdown infringement after four minutes, but Biggar responded with one of his own to level matters.
The Scarlets' fly-half added a second three-pointer as the home side regained their advantage before the away side's stand-off once again got the Ospreys back into the game.
Biggar then scored his third from the tee to put his side into a 9-6 half-time lead.
There wasn't much change in the second half either with the match remaining attritional and tight.
Priestland's early penalty was all either team had to show for most of the period.
However, the Ospreys hadn't read the script and went over for two late touchdowns from Jones and Dirksen to seal the victory.
North did score late on for the hosts to rescue a bonus-point but it wasn't enough.