The figure is slightly down from a similar poll last year, but makes it the second year that a majority of Americans have supported same-sex marriage, the poll revealed.
The poll comes as North Carolina voted in favour of a gay-marriage ban, defining marriage as purely union between a man and a woman.
President Barack Obama, who has stopped short of supporting same-sex marriage, but has described his position on the issue as evolving, faces mounting pressure to change his stance, especially as his top aides come out in favor of legalisation.
Over the weekend, Vice President Joe Biden said he is "absolutely comfortable" with same-sex marriage, and Education Secretary Arne Duncan later said publicly that gay marriage should be legal.
When Gallup first asked the American public about gay marriage in 1996, only 27 percent of respondents supported it.