The poll, released Friday, found 38 percent of U.S. Catholics oppose same-sex marriage. A December poll indicate American Catholics supported allowing same-sex marriage 49 percent to 43 percent.
Americans overall support same-sex marriage by 47 percent to 43 percent. A 2008 survey found 55 percent of Americans opposed it.
"Catholic voters are leading American voters toward support for same-sex marriage," said Peter A. Brown, assistant director of the Quinnipiac University Polling Institute, in a statement.
The poll found 52 percent of American Catholics think church leaders are out of touch with the prevailing views of the faithful, Politico noted.
The telephone poll of 497 adult Catholics was conducted Feb. 27-Mar. 4 and has a margin of error of 4.4 percentage points. The same-sex marriage question was asked of 1,944 registered voters nationwide, with a margin of error 2.2 percentage points.