Not only was it a fine response for Scarlets' 17-15 defeat at the Liberty Stadium last weekend, but the victory was also their first over Ospreys since Boxing Day 2011.
Ospreys were undone by tries from Liam Williams and John Barclay, while Rhys Priestland kicked over 12 points. All Ospreys could muster was a try from Dan Evans and a penalty and conversion from the boot of Dan Biggar.
The defeat also ended a run of 17 wins for the Ospreys against all the other Welsh regions going back to 2011.
It did not help the visitors that they were forced into a significant late change when influential Wales scrum-half Rhys Webb withdrew through illness just before kick-off. He was replaced by Martin Roberts.
The only other change for Ospreys saw wing Aisea Natoga replace Eli Walker.
Wales centre Scott Williams returned to captain the Scarlets after recovering from a shoulder injury, with Hadleigh Parkes moving out to the wing as Kristian Phillips banned for three games after a tackle on Dan Evans in the reverse fixture last week.
In a feisty first half the Scarlets showed huge desire to avenge the previous weekend's loss at the Liberty Stadium and deserved their 14-3 lead at the break.
The presence of Scott Williams paid dividends almost immediately as his break forced Ospreys offside with Priestland converting the subsequent penalty goal in the third minute.
The skipper was in the thick of the action and was probably unfortunate to be sin-binned on 11 minutes for a high tackle on Scott Baldwin.
While he was off the field Biggar put over a penalty from a Scarlets offence at the breakdown.
But after that Scarlets dominated the rest of the half and after a second Preistland penalty the home side scored the game's opening try through Liam Williams, who went over after a fine break by Priestland.
Just before half time an Ospreys collapsed scrum on their own line and Priestland punished them with a third penalty.
It did not get any better for the Ospreys in the opening minute of the second half as they were on the end of a fine try created from a burst by Scott Williams and scored by Barclay after good work by Liam Williams.
A third Priestland penalty opened up a 19-point advantage for the Scarlets after another strong scrum from the home region.
Thirteen minutes from time full-back Evans grabbed a try back for the Ospreys, converted by Biggar, but any momentum they had built up was snuffed out by resilient defence from the Scarlets in the final 10 minutes.