Ulster jumped to sixth place in the URC table, after a four try bonus point victory in Wales against Dwayne Peel's Scarlets side.
Ulster trailed 6-3 at half time, after being thwarted by Scarlets defence on three occasions, holding up three potential tries. Sam Costelow stuck away two penalties for the Welsh side, with John Cooney scoring one for the away side.
After half-time, Ulster punished Scarlets with four converted tries through Stuart McCloskey, Nick Timoney, Dave McCann and Jacob Stockdale, securing the bonus-point for Ulster.
Scarlets didn't make it an easy half themselves, with three tries of their own as they pushed Ulster all the way in Llanelli.
Richie Murphy's side went into the game knowing a win would be vital as Ulster hint down the play-off positions, starting the match in seventh position.
It was a special occasion for Mike Lowry, the flying full-back making his 100th appearance for his province and leading the side out.
In a cagey opening start to the game, Scarlets took advantage of penalties to put themselves six points to the good.
Ulster had their moments in the half, with Cormac Izuchukwu, starting for the first time at blindside flanker, being thwarted three times in the half by the Scarlets holding the ball up on the line.
It was an impressive rearguard effort, as the Welshmen were down to 13 men at the end of the half, after yellow cards to Plumtree and Davies after their offences.
Ulster went into the break frustrated, but knew they had showed enough attacking threat to score in the second half.
HT: Scarlets 6-3 Ulster
After the break, Ulster scored their first try of the match, after Stuart McCloskey powered through a gap in the Scarlets defence. John Cooney converted to send Ulster into a 6-10 lead.
Minutes after Ulster had their second as Scarlets looked to attack. Mike Lowry intercepted a pass to sprint into the Scarlets territory and from the scrum 5m out from the Scarlets line, Nick Timoney bulldozed over for Ulster's second.
David McCann gave the province more breathing space as the in-form flanker finished off attacking pressure from close range.
With the score at 6-24, Ulster were chasing a bonus try. However the province were pegged back a little by Scarlets, who were keen to put on a show for the team's final home game of the season.
An attack saw their eight, Carwyn Tuipulotu, ground for the home side despite Ulster's efforts.
Fitting with the back and forth nature of the contest, Ulster got their bonus-point minutes later, with Jacob Stockdale providing good width to Ulster's attack and running through to score on the wing for his eighth URC try of the campaign.
It was just what the winger deserved after an impressive high ball catch from the restart.
Scarlets reduced the scoreline to 11 points, after Tomi Lewis ran through for a converted try, but Ulster held on well in the closing 10 minutes, with well-worked lineouts from Tom Stewart under pressure and the defence showing good discipline.
When the final whistle went, Ulster's players and staff went home happy after the job was done.
Ulster came for a 5 point win and got exactly that, with the province firmly in the fight for play-off positions, as they face interprovincial rivals, Leinster Rugby, in the final home match of the regular season next Saturday.
FT: Scarlets 20-31 Ulster
Player of the Match - Stuart McCloskey
Scorers
Ulster
TRY: McCloskey, Timoney, McCann, Stockdale
CON: Cooney x4
PEN: Cooney
Scarlets
TRY: Tuipulotu, Lewis
CON: Costelow x2
PEN: Costelow x2
Teams
Ulster
(15-9) Mike Lowry, Ethan McIlroy, Will Addison, Stuart McCloskey, Jacob Stockdale, Billy Burns, John Cooney;
(1-8) Eric O’Sullivan, Rob Herring, Scott Wilson, Kieran Treadwell, Alan O’Connor (C), Cormac Izuchukwu, David McCann, Nick Timoney.
Replacements: Tom Stewart (Rob Herring), Andrew Warwick (James French), James French (Scott Wilson), Harry Sheridan (Kieran Treadwell), Reuben Crothers (Nick Timoney), Nathan Doak (John Cooney), Stewart Moore (Stuart McCloskey), Dave Ewers (Cormac Izuchukwu).
Scarlets:
(15-9) Ioan Nicholas; Tomi Lewis, Johnny Williams, Eddie James, Ryan Conbeer; Sam Costelow, Gareth Davies;
(1-8) Kemsley Mathias, Ryan Elias (capt), Harri O’Connor, Morgan Jones, Jac Price, Taine Plumtree, Dan Davis, Carwyn Tuipulotu.
Replacements: Shaun Evans, Wyn Jones, Sam Wainwright, Ben Williams, Jarrod Taylor, Kieran Hardy, Ioan Lloyd, Scott Williams.