Treviso's Kristopher Burton scored 18 points to send Ulster to their third successive league defeat and a first loss at Ravenhill since last December's defeat to Leinster.
Forward dominance gave Treviso the edge throughout and a penalty try at the end of the first half, along with Burton's intercept score after 67 minutes, gave Ulster no hope of clawing the game back despite two late tries from Pedrie Wannenburg.
Burton opened the visitors' account with a sixth-minute penalty and after Jared Payne was helped from the field with what appeared to be an ankle injury, the home side's game imploded for the rest of the half.
Burton was wide with a long-range effort before Treviso, who were already on top at the breakdown, managed to put serious pressure on the Ulster scrum.
A loose kick from Humphreys saw winger Brendan Williams step his way through the Ulster defence before being hauled down just short of the line.
That led to a series of scrums on Ulster's line which, after Italian referee Stefano Penne had sin-binned prop Declan Fitzpatrick, ultimately led to a penalty try in the 41st minute, which Burton converted.
The early exchanges of the second half saw Burton short with a 40-metre penalty and then, with Fitzpatrick back on the field, Humphreys was wide with an effort from a similar range.
However, after Ulster pulled down an advancing Treviso maul, Burton made no mistake with his second penalty just before the hour mark.
It got worse for the home side as replacement full-back Adam D'Arcy's kick through landed in Burton's arms and the fly-half ran in unopposed from just over halfway. Burton's conversion gave his side a 20-0 lead after just over an hour.
Ulster needed a quick response and almost their first decent attack of the game saw Wannenburg put over at close range from a Paul Marshall pass. Humphreys converted to reduce the deficit to 13 points with just over 15 minutes remaining.
Wannenburg crossed the line again on 73 minutes from a lineout, but Humphreys could not land the extras and Ulster remained eight points adrift and just outside bonus point range.
Burton then finished the scoring with an injury-time penalty.