Replacement Josh Macleod scored the decisive try in the added minutes as Scarlets completed a 12-9 victory over Ospreys in their Pro 14 encounter in Llanelli on Tuesday.
Northampton Saints suffered their ninth defeat in 10 games spread across all the competitions after they were beaten 32-43 by Ospreys in the latest round of European Champions Cup at Franklin's Gardens on Saturday.
Ospreys have confirmed that Wales international Aled Davies will join the Pro 14 club from Scarlets ahead of the next season after signing a two-year contract.
Ospreys have finally managed to end their seven match losing streak across all the competitions with a 28-14 triumph over Dragons in their Pro 14 clash in Swansea on Friday.
Wales international Scott Baldwin feared that he had lost his left hand after he was bitten by a lion during the recent tour of South Africa last month.
A second half double from Dan Evans did not prove enough as Ospreys lost the European Champions Cup encounter against Clermont Auvergne with a 21-26 margin at Liberty Stadium on Sunday.
Munster managed to maintain their unbeaten streak in the ongoing season of Pro 14 as they overcame Ospreys with a 16-21 triumph in their Pro 14 clash in Swansea.
Alex Dunbar scored a double while Finn Russell kicked 11 points as Glasgow Warriors secured a 31-10 bonus point victory over Ospreys in their Pro 14 clash at Scotstoun Stadium.
England international Manu Tuilagi made his first return in eight months for his club Leicester Tigers' during the pre-season friendly clash against Ospreys on Friday.
Munster came up with a three try performance as they went on to defeat Ospreys by 23-3 in the semi final of Pro 12 at Limerick to set up a clash in the finals with Scarlets next weekend.
Stade Francais successfully overcame a sin-bin and a red card of Josaia Raisuqe to win the European Challenge Cup quarter-final against Ospreys at Principality Stadium on Sunday.