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Scrum-half Luke McGrath snapped up two first-half tries but the visitors were just 12-9 down at the break thanks to three penalties by Dan Jones.
Leinster broke clear in the second half at the RDS with Rhys Ruddock crashing over and Joey Carbery scoring twice.
Replacement Ed Byrne also got over and Jamison Gibson-Park came on to score a try with the last move of the match.
The five-point return takes Leo Cullen's men two points clear of the Ospreys who had gone top on Friday with victory in Edinburgh.
Scarlets were unchanged from their first win over Munster at Thomond Park for 14 years, but their hopes of an Irish double were blown away in the second half as Leinster coasted to their fourth successive bonus-point victory of the Six Nations period.
After Jones had kicked the visitors ahead early on, Leinster turned down a kickable penalty and from a close-in scrum, McGrath scrambled over for the opening try.
A second successful penalty followed from Jones, but the Scarlets fly-half then blundered when a was charged down by McGrath, who finished smartly for his second try.
The Welsh club were just three points in arrears at half-time but conceded 33 without reply after the interval.
Leinster ran in four tries during the third quarter - Ruddock charging through and full-back Carbery snapping up two and replacement Ed Byrne also scoring.
Just to rub it in at the death, another replacement, Gibson-Park, completed the rout for Leinster, profiting from Dan Leavy's barnstorming run towards the right corner.
Leinster: Carbery, A. Byrne, Kirchner, Reid, McFadden, R. Byrne, L. McGrath, Dooley, Strauss, Bent, Molony, Triggs, Ruddock, Leavy, Conan.
Replacements: B. Daly for Reid (56), Gibson-Park for L. McGrath (60), B. Byrne for Dooley (57), E. Byrne for Strauss (57), Ross for Bent (49), M. McCarthy for Triggs (60), Deegan for Ruddock (68).
Scarlets: McNicholl, T. Williams, S. Hughes, Parkes, van der Merwe, D. Jones, J. Evans, W. Jones, Elias, Kruger, Price, Beirne, Shingler, J. Davies, Boyde.
Replacements: Nicholas for S. Hughes (45), A. Thomas for D. Jones (56), D. Smith for J. Evans (79), Garrett for W. Jones (68), D. Hughes for Elias (69), N. Thomas for Kruger (49), Bernardo for Price (60), Allen for J. Davies (49).
Referee: Nigel Owens (Wales)
Leinster returned to the top of the Pro12 table and bagged a bonus point with an emphatic win over the Scarlets.