Initial results show North Carolina has approved a constitutional amendment effectively banning same-sex marriage or civil unions.
State election officials said 61% had voted for the measure banning gay unions, and 39% opposed it.
Recent polling had suggested the ban would pass, defining marriage as only between a man and a woman.
North Carolina law already bans gay marriage but the vote enshrines this in the constitution.
Recent comments in favour of gay marriage by US Vice-President Joe Biden had reignited debate over the issue.