Ulster ended the Scarlets' run of eight successive wins in all competitions with a Pro12 victory in difficult wet and windy conditions at Ravenhill.
Sean Lamont and Viliame Iongi scored early tries for the Scarlets but Ian Humphreys kicked a penalty and Stephen Ferris scored a try before half-time.
Humphreys added three further penalties after the break and then converted Ferris's second try of the game.
A late Kieran Murphy try and Stephen Jones conversion earned a bonus point.
The Scarlets began the game without several key players who were on Welsh international duty, while Ulster welcomed back a number of their frontline figures.
Winger Lamont crossed the Ulster line after two minutes when he intercepted a Pedrie Wannenburg pass to dive over in the corner.
Jones missed the conversion and then Humphreys was off-target with his first kick at the posts after the home side were awarded a penalty.
Scarlets continued their impressive start to the match when Tongan international Viliame Iongi shrugged off challenges from Darren Cave and Craig Gilroy to touch down for his side's second try after thr backline went through the phases in the 14th minute.
Jones was unable to add the additional two points.
A Humphreys penalty reduced the Irish province's deficit and then Nevin Spence threatened to score but play was pulled up for a forward pass by Andrew Trimble.
The visitors were reduced to 14 men for 10 minutes when Daniel Evans was sent to the sin bin for deliberate obstruction from an offside position on 31 minutes.
Man-of-the-match Ferris dived over for a try after a break and chip kick from Paul Marshall set up the chance.
Darren Cave and Humphreys also played their part in the move, but the Ulster fly-half was unable to convert.
Humphreys knocked over three penalties in the third quarter of the game, the third from long range.
Ferris dummied a pass and barged through the Scarlets defence for his second touchdown, Humphreys adding the conversion on this occasion.
Ulster prop Adam Macklin received a yellow card for slowing the ball down within minutes of entering the fray and the Scarlets took advantage by finishing strongly.
Murphy went in under the posts after poor Ulster defence and Jones landed the simple conversion.
Ulster: A D'Arcy; A Trimble, D Cave, N Spence, C Gilroy; I Humphreys, P Marshall; T Court, R Best, J Afoa, J Muller (capt), D Tuohy, S Ferris, C Henry, P Wannenburg.
Replacements: N Brady for Best (70), P McAllister for Court (54), A Macklin for Afoa (70), L Stevenson for Tuohy (54), R Diack for Henry (70), R Pienaar for Marshall (63), P Jackson for Humphreys (70), I Whitten for Spence (60)
Scarlets: D Evans, V Iongi, N Reynolds, A Warren, S Lamont, S Jones (capt), G Davies, P John, K Owens, D Manu, L Reed, A Powell, M Gilbert, R McCusker, K Murphy
Replacements: E Phillips for Owens (57), I Thomas for John (59), S Gardiner for Manu (66), D Welch for Powell (57), J Edwards for Gilbert (48), D Newton for D Evans (66), I Evans for Warren (39).
Not used: R Williams
Referee: Alain Rolland (Ireland)