Mistakes were made in the opening 20 minutes when the home side gave away a penalty, Owen Williams kicking the visitors ahead on 4 minutes.
But Ruan Pienaar levelled the game with a penalty on 6 minutes and a second penalty attempt by Williams hit the bar.
It was in the 25th minute the men in white began to take control - Tommy Bowe brought home a try in the corner after a beautiful pass from Pienaar, who then kicked the conversion to make it 10-3.
Ulster really started to assert some authority on the pitch with a second try from scoring machine Robbie Diack.
Unfortunately Ruan Pienaar missed the conversion but another penalty from the South African secured the first half, leaving the table-toppers dominant at 18-3.
Half time: Ulster 18-3 Scarlets
Tom Court opened the second half with a third Ulster try, ramping up the scoreline to 23-3 before Pienaar added the extra 2 points.
A try from Scarlets' Gareth Davies, converted by Owen Williams, claimed 7 points and signalled the start of a shaky second half for Ulster.
Ruan Pienaar kicked another penalty from right in front of the posts, making it 28-10.
The visitors then made a respectable attempt to drive home more points, Timani crossing for Scarlets' second try and Williams adding the conversion.
But it wasn't enough and Ravenhill erupted with the final whistle, Ulster finishing 15 points clear of the Welsh side and claiming their place in the final.
Full time: Ulster 28-17 Scarlets
The game marked the end of an era at Ravenhill as Ulster fans bid farewell to the ground's historic grandstand.
Ulster hooker and Man of the Match Rory Best, who admitted he’ll miss the atmosphere created by the old grandstand, said: “Ultimately this is where we want to be.
“We want to win silverware this year and this is our last crack at it now.
“This team, we felt at the start of the season, was good enough to win something — we wanted to win everything, but this is our thing now and we’re delighted to be in the final.”
Ulster will play the winner of tomorrow's Leinster v Glasgow match in Dublin on Saturday, May 25.
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