The Gallup poll found 53 percent oppose same-sex marriage and 44 percent favor allowing it.
The polling firm said 68 percent were opposed and 27 percent in favor when it first surveyed Americans about gay marriage. The latest numbers, Gallup said, were tied for the lowest opposition but slightly below the highest ever support -- 46 percent -- in 2007.
Among Democrats questioned, a majority favor legal gay marriage, with 70 percent identifying themselves as liberals expressing support.
Those surveyed who say religion is "very important" in their lives oppose legal same-sex marriage by 70 percent to 27 percent. On the other hand, those who say religion is not important to them support gay marriage by just as wide a margin, Gallup found.
The telephone poll of 1,029 adults was conducted May 3-6 and has a margin of error of plus or minus 4 percentage points.