A second-half comeback from Scarlets condemned champions Connacht to a 17-8 loss in Wales, their third consecutive Pro12 defeat.
Tries from Liam Williams either side of the break helped Scarlets bounce back from 8-7 down at the break and register their first win of the season.
Connacht are still looking for their first league point of this campaign and are rooted to the foot of the table.
Williams went over either side of half-time with both tries converted by outside-half Rhys Patchell, who added a drop-goal. Connacht's try came from wing Niyi Adeolokun with Jack Carty kicking a penalty.
This was a bottom-of-the-table clash, with Scarlets having lost their opening three games of the season and holders Connacht having suffered heavy home defeats against Glasgow and Ospreys.
Connacht made seven changes from the team that played in the Zebre game which got abandoned because of torrential rain, with Irish internationals Matt Healy, Kieran Marmion, Finlay Bealham, Quinn Roux and Ultan Dillane all starting.
For Scarlets, Aaron Shingler returned at blindside flanker with Aled Davies coming in at scrum-half to replace Gareth Davies, who had a virus.
The inclement weather in Llanelli did not help two sides low on confidence.
Connacht just about edged a mistake-ridden first half with Scarlets' cause not helped by Aled Thomas' wayward kicking, the full-back missing three penalties inside the first 20 minutes.
Those misses proved crucial as centre Bundee Aki broke into Scarlets' 22 and when the ball was moved right, Adeolokun went over in the corner. Carty missed the conversion.
Matters did not get much better for Scarlets when Patchell kicked his penalty attempt to the corner into the dead ball area.
But Carty missed a penalty attempt at the other end before Scarlets broke their duck to go into the lead briefly when wing Williams ran in to finish off a move begun by a Shingler break, with Patchell converting.
Their 7-5 lead only lasted three minutes before Carty kicked a final-minute penalty after skipper Ken Owens had disrupted a Connacht maul illegally.
In the second half, wing Steff Evans went closest to scoring a try but with Scarlets still trailing the Welsh region brought on Scott Williams in a reorganised backline.
Scarlets began to play on the front foot with scrum-half Aled Davies and Owens combining well before Williams went over in the left corner.
The conversion was put over from the touchline by Patchell, who also kicked a 71st minute drop-goal to deny Connacht a losing bonus point.
