Leo Cullen's side overpowered its Pro14 rival, scoring five tries in an imperious performance to set up a showdown with either Racing 92 or Munster in Bilbao on 13 May (AEST).
The three-time European champion maintained its unbeaten record in the competition in ruthless fashion, James Ryan, Cian Healy and Fergus McFadden going over for first-half tries in Dublin.
Leinster was relentless, the outstanding Scott Fardy and Jonathan Sexton adding further tries in the second half as Scarlets was unable to contain the favourite, missing out on a first European final.
Sexton scored 13 points from the tee and three first-half penalties from Leigh Halfpenny were all Wayne Pivac's men could come up with before Tadhg Beirne scored their solitary try right at the end.
The Scarlets took an early lead when Halfpenny nailed a penalty from around 45 metres out, but they were soon behind when Leinster produced a brilliant move for the opening try.
Rhys Patchell produced a great last-ditch tackle to prevent Isa Nacewa from going over in the corner after Leinster had moved the ball swiftly from left to right after a lineout, but Ryan powered his way across the line.
Sexton converted and added a penalty as the Scarlets came under immense pressure before Halfpenny made it two from two at the other end.
It was only a matter of time before Leinster crossed the line for a second time, though, and it was Healy who dived over after the persistent home side had gone through phase after phase.
Halfpenny was on target with a third penalty, but Leinster was rampant and McFadden was left with a simple finish after waves of powerful carries, Sexton adding the extras to make it 24-9 at the break.
McFadden was unable to return for the second half because of an injury suffered while applying the finish, but Leinster continued where it left off after the interval, the excellent Fardy diving over from a give-and-go with Ryan.
Leinster showed no mercy on the Welsh side, Sexton taking a pass from Jamison Gibson-Park and creating space for himself with a side-step before darting through a gap to score his side's fifth try and making no mistake with the conversion.
Beirne burst through for a consolation try late on against his former club, but the Leinster supporters had long since started celebrating a trip to Spain as it stands on brink of being crowned European champion for the first time in six years.