Not since the 2010-2011 season had Treviso beaten an Irish side, but it pulled off a big upset at RDS Arena a week before Leo Cullen's men face the Scarlets in the semi-final of the European Champions Cup.
Richardt Strauss's early try put Leinster on course to seal a home semi-final and Barry Daly powered his way over in the corner just before half-time to put them 12-5 up after Luca Bigi got the Italian side on the board with a five-pointer.
Federico Ruzza scampered away for a second Treviso try and the influential Tommaso Allan touched down to put the visitors in front after booting the loose ball forward to punish a stray pass from Jack McGrath.
Joey Carbery was on target with a penalty just before the hour-mark, but Treviso held on for a famous win, the fit-again Sean O'Brien and Jordan Larmour unable to make an impact in a poor display from Leinster.
An under-strength Scarlets side was put to the sword by Edinburgh, Duhan van der Merwe scoring a brace of tries as Richard Cockerill's men took a giant stride towards qualification for the final series.
The Scarlets, missing key players with the Champions Cup last-four taking priority, conceded three tries in the first 20 minutes and four in the second half, Sam Hidalgo-Clyne racing 50 metres to go over for one of their eight tries and also kicking 10 points.
Jared Rosser's hat-trick was in vain as Zebre beat the Dragons 34-32, while Cardiff Blues was an emphatic 45-12 winner at Southern Kings.