Kris Burton was again impressive from the kicking tee with ten points, missing just one shot at goal, while outside backs Ludovico Nitoglia and Brendan Williams scored the decisive tries.
Liam Williams' second-half try along with five points from full-back Dan Newton were not enough for the Scarlets who slip to their fourth defeat in five games.
It was the visitors who had made the brighter start but Newton missed an early penalty attempt.
Burton let him off the hook with a penalty miss of his own but after Newton had broken the deadlock on the half-hour, Rhys Thomas was sent to the bin for coming in from the side.
The home side took full advantage with Burton knocking over the resulting penalty before Nitoglia grabbed the game's opening try.
Williams sparked the move, making inroads into the Scarlets' defence before finding his full-back to cross and hand the Italians a 10-3 lead on the stroke of half-time.
The Scarlets bounced back immediately at the start of the second half as Lee Williams served namesake Liam who crossed and with Newton's conversion the sides were level.
Burton added another penalty to put Treviso back in front and Brendan Williams' interception try put them in the clear, Burton again adding the extras.
Newton had the chance with 20 minutes to go to bring the Scarlets back to within seven points but his penalty attempt drifted wide.
And despite their best efforts Davies' team could not find a way through the Treviso defence as the Italians repeated last season's win in the corresponding fixture.
|Benetton Treviso Score Card|
|Scarlets Score Card|