The Welsh province lost its crown to Leinster in a thrilling contest in Dublin last May, but was back on top with a 23-21 victory in this latest encounter.
Ken Owens scored an early Scarlets try, after which the hosts relied largely on the boot of Leigh Halfpenny to stay in touch after Fergus McFadden and James Lowe crossed.
Gareth Davies put the game beyond Leinster, with Rhys Ruddock's try five minutes from time not enough to rescue a result.
Elsewhere, Benetton Treviso snatched a dramatic 27-25 victory against Cardiff Blues as Tommaso Allan kept his nerve for a match-winning conversion after an 83rd-minute try.
Allan scored the first try of the match, but Treviso gifted the visitors a penalty try before they touched down again through Braam Steyn.
Although Cardiff's only other points came with the boot, Gareth Anscombe's trio of second-half penalties looked to be enough for victory until Monty Ioane dramatically crossed and Allan delivered.
Ospreys continued its winning start to the season in style, defeating Cheetahs 46-14.
Alun Wyn Jones crossed at either end of the match for the dominant hosts, with Justin Tipuric also scoring a late brace among seven tries for the Ospreys.
Josh Lewis inspired Dragons to a 27-22 defeat of Southern Kings, scoring a crucial 17 points, while Connacht eased to a 32-13 win over Zebre.