Results of The Washington Post-ABC News poll released Monday indicate 36 percent said marriage for same-sex couples should be illegal.
Monday's poll results were a near-reverse of a poll 10 years ago when 37 percent favored same-sex marriage 55 percent opposed it, the Post said.
The Supreme Court takes up the question of same-sex marriage next week.
Former Secretary of State Hillary Clinton said she supported same-sex marriage Monday in a video posted by Human Rights Campaign.
Nearly two-thirds of all Americans said the question should be decided for all states based on the U.S. Constitution, not each state making its own laws, results indicated.
Among young adults in the 18-to-29 age bracket, support for gay marriage is overwhelming, reaching a record 81 percent in the new poll. While support has increased among older adults, those aged 65 years and older remain opposed, with 44 percent saying same-sex marriage should be legal and 50 percent saying it should not.
Sixty-two percent of Americans said being gay is the way some people are, not something people choose to be, results indicated.
Currently, same-sex marriage is legal in nine states and the District of Columbia.
Results are based on nationwide phone interviews with 1,001 adults conducted March 7-10. The margin of error is 3.5 percentage points.