Twenty-eight other state have voter-approved constitutional bans on same-sex marriages, according to the National Conference of State Legislatures.
Massachusetts, Iowa, Vermont, New Hampshire, Connecticut, New York and the District of Columbia allow gay and lesbian nuptials.
Maryland, New Jersey and Washington state have passed laws this year approving same-sex marriage, but Governor Chris Christie vetoed New Jersey's law and opponents in Maryland and Washington are threatening ballot initiatives to overturn those laws.
The vote in North Carolina followed statements by senior officials of President Barack Obama's administration this week which were interpreted as supporting gay marriage.
Obama has said he favors civil unions but has stopped short of supporting gay marriage.
Supporters of the amendment in North Carolina, a swing state in the Nov. 6 presidential election, said it would preserve the sanctity of marriage between a man and a woman and make laws forbidding gay marriage harder to repeal.