Tyler Morgan starred in only his second league start, scoring one try and setting up another for Will Harries, with 11 crucial points coming from the boot of Jason Tovey.
Fabio Semzenato and Manoa Vosawai crossed to bag the Italians a losing bonus-point which gave them a spot in next year’s Rugby Champions Cup.
The Dragons stormed into the lead after just three minutes. Richie Rees ran a wide arc from the base of a ruck and offloaded to the onrushing Pat Leach before Treviso slowed the ball down illegally.
Jason Tovey, making his 100th start for the Dragons, slotted his first penalty from in-front of the posts.
But Treviso hit-back after quarter of an hour through scrum-half Semenzato. Their pack drove from half-way into Dragons territory before the quick-witted Italian darted past the defence to score at the corner.
They almost had another one after Ludovico Nitoglia broke clean through and fed flanker Christian Loamanu.
But the Japanese international was stopped just short, Will Harries then refused to release the ball, and was duly sin-binned.
The Italians did stretch their lead shortly after through Alberto Di Bernardo, the Dragons played the ball on the floor and the fly-half beat the wind to take the three points.
Morgan, on only his second league start, then cut a stunning angle to put the Dragons back in-front. Lewis Evans delivered a flat pass which the 18-year-old hit at full speed and kept going all the way to line.
Di Bernardo kicked another penalty before half-time to put his side a one point advantage at the break.
Morgan showed his promise once again four minutes after the interval. The schoolboy made a break down the touchline and chipped ahead for Harries who collected to claim the five points.
But Treviso claimed their European Champions Cup place after Cardiff Blues signing Vasawai crashed over from a close range line-out and Di Bernardo hit another penalty.
