|Leinster (9) 19|
|Try: Ross Con: Madigan Pens: Madigan 4|
|Scarlets (15) 15|
|Tries: Parkes, Owen Con: D Jones Pen: D Jones|
Scarlets had their 100% Pro12 record ended by Leinster at the RDS in Dublin.
Hadleigh Parkes and Gareth Owen went over for the Welsh side in the first half and Scarlets were good value for their 15-9 lead at half-time.
But three Ian Madigan penalties kept Leinster within touching distance and they dominated the second half.
Mike Ross' close-range try and Madigan's conversion soon after the restart put Leinster into the lead and another Madigan penalty capped the win.
Leinster coach Leo Cullen had named a completely new starting XV for the game against the league leaders, his options boosted by the return of half-a-dozen Ireland internationals.
Scarlets also had returning World Cup stars, with scrum-half Gareth Davies returning to the fold after Wales duty.
The Welsh side took an early grip with a series of drives at the Leinster line that allowed centre Parkes the space to barrel over, his try converted by Dan Jones.
Leinster responded to put Scarlets under pressure, with lock Jake Ball sin-binned for overstepping the boundaries during some frantic defence.
In his absence Leinster closed the gap with two Madigan penalties, although they lost Rob Kearney to injury and there could be a nervous wait to see if the Ireland full-back is fit for Europe next weekend.
Back to full strength, Scarlets stretched away again as number eight John Barclay joined the line to put Owen away for an unconverted try.
Madigan added a third penalty but Jones responded in kind to ensure Scarlets took a six-point lead into the break, although it could have been more with Aled Thomas wide with a long-range kick.
As well as Scarlets had played, they found themselves behind for the first time within five minutes of the restart as Ross went under almost under the posts for a converted try after a series of forward drives.
Johnny Sexton come off the bench for the home side, with the Ireland fly-half slotting in at centre outside Madigan.
The combination almost produced a decisive try as Sexton loped away to the line, only to be denied by a crossing call confirmed by video replays.
But Madigan added his fourth penalty to give Leinster a four-point cushion, although Scarlets at least had the consolation of a losing bonus point despite being dominated in the second half.
Scarlets coach Wayne Pivac told BBC Radio Wales: "I'm very disappointed, I thought the boys played very well. I'm disappointed the breakdown was refereed one way, I thought.
"The stream of penalties against us [made the difference], I thought we got into some really good positions over the ball and every other week we're getting penalties for that.
"From where I was sitting, we got into some great positions for turnovers and didn't get the rewards... It should have been seven wins.
"I don't think it takes anything out [of the season's momentum], the boys know they can compete with these sides now away from home.
"That was the pleasing side of it, that we had an opportunity to win the game at the end but we probably blew it really. We got in behind them when Maselino [Paulino] had the big break and we bombed a try."
Leinster: Rob Kearney; Zane Kirchner, Garry Ringrose, Noel Reid, Cian Kelleher; Ian Madigan, Isaac Boss; Cian Healy, James Tracy, Mike Ross, Devin Toner, Tom Denton, Dominic Ryan, Sean O'Brien, Jamie Heaslip (capt).
Replacements: Bryan Byrne, Jack McGrath, Marty Moore, Hayden Triggs, Dan Leavy, Luke McGrath, Johnny Sexton, Ben Te'o.
Scarlets: Aled Thomas; Tom Williams, Gareth Owen, Hadleigh Parkes, Michael Tagicakibau; Dan Jones, Gareth Davies, Rob Evans, Emyr Phillips, Peter Edwards, Jake Ball, Lewis Rawlins, Aaron Shingler, James Davies, John Barclay (capt).
Replacements: Ken Owens, Dylan Evans, Will Taylor, Tom Price, Maselino Paulino, Aled Davies, Michael Collins, Harry Robinson.
Referee: Leighton Hodges (WRU).
Assistant referees: Leo Colgan, Olly Hodges (IRFU).
Citing commissioner: Wayne Sheridan (IRFU).
TMO: Seamus Flannery (IRFU).