The Quinnipiac University poll found 54 percent of Catholics support same-sex marriage while just 38 percent are opposed, compared to a 47-43 percent margin among all American voters.
Both margins represent a reversal from the 36-55 percent opposition among all voters the group found as recently as July 2008.
"Catholic voters are leading American voters toward support for same-sex marriage," said Peter Brown, assistant director of the polling institute.
US Catholics are more ambivalent when its comes to the Church, with 52 percent saying it is moving in the right direction, while the same percentage says Church leaders are out of touch with their views.
Fifty-five percent said the next pope should move the Church in a new direction, while 38 percent said he should maintain the Church as it is.
"Looking at all adult Catholics, different from the wider survey of all registered voters, we see a conflicted group," Brown said.
Those surveyed said priests should be allowed to marry by a margin of 62-30 percent and 64 percent said the next pope should relax rules on contraception, compared to 28 percent who were opposed.
Eighty-one percent of Catholic respondents said the Church should do more to combat child sex abuse by priests.
Quinnipiac surveyed 497 Catholic adults from February 27 to March 4, with a margin of error of 4.4 percent.
The same-sex marriage question was asked of 1,944 registered voters nationwide, with a margin of error of 2.2 percent.