Replacement Liam Davies darted through a gap in the Italian defence with four minutes to go to score the Scarlets' all-important fourth try and seal both the win and the bonus point, moving them to within one point of fourth-placed Glasgow.
Treviso, who were without 15 first-choice players, had broken the deadlock as early as the second minute when Ludovico Nitoglia went over in the corner.
The visitors threw long from a lineout and Simon Picone found Nitoglia on his inside and the full-back escaped Andy Fenby to go over in the corner. Willem de Waal knocked over the difficult conversion from the right touchline.
Things got worse for the Scarlets soon after as Aled Thomas took too long with his clearance kick and Treviso second row Valerio Bernabò charged it down before collecting to dot down next to the posts, de Waal again converting.
Thomas had the chance to make amends on the quarter hour with a penalty but dragged his effort wide of to the left.
Just before the half hour the Scarlets finally got themselves back into the game after a quick hands from Emyr Phillips after a ruck allowed Josh Turnbull to burst through a stretched Treviso defensive line, Thomas added the extras to make it 14-7.
And the Scarlets got their second try three minutes before the break when Deacon Manu dived over from close range after a powerful five metre scrum, Thomas converting to level the scores.
Treviso did manage the final say in the half when de Waal knocked over a long-range penalty to make it 17-14 at the break.
The home side had the chance to draw level ten minutes into the second half but Thomas missed his second kick of the night.
However the Scarlets were not to be denied for long as the Treviso defence finally wilted with Liam Williams crashing through the tackle of Brendan Williams for their third try of the night.
Thomas was back on target with the conversion to make it 21-17 to the home side before punishing a further Treviso indiscretion to stretch the lead to seven with a quarter of the game remaining.
Things got worse for the visitors soon after when Treviso replacement Gonzalo Garcia was sin-binned almost as soon as he came on for tipping over Kieran Murphy.
However Aaron Shingler soon followed him to the bin as the home side eventually paid the price for their lack of discipline, allowing de Waal to make it 24-20 with just over ten minutes to go.
That sparked the Scarlets into life and they earned a penalty with their next attack, allowing Thomas to extend the lead once more.
And with four minutes remaining replacement scrum-half Davies darted over for the final try, Thomas again added the extras.
|Scarlets Score Card|
|Benetton Treviso Score Card|
|Willem de Waal||2||2||10|