Unofficial returns showed the amendment passing with about 61 percent of the vote to 39 percent against.
North Carolina is the 30th state to adopt such a ban on gay marriage.
In the final days before the vote, members of President Barack Obama's cabinet expressed support for gay marriage and former President Bill Clinton recorded phone messages urging voters to oppose the amendment.
Supporters of the amendment responded with marches, television ads and speeches. The Rev. Billy Graham was featured in full-page newspaper ads backing the amendment.
North Carolina law already bans gay marriage, but an amendment effectively seals the door on same-sex marriages.
The amendment also goes beyond state law by voiding other types of domestic unions from carrying legal status, which opponents warn could disrupt protection orders for unmarried couples.
Both sides spent a combined $3 million on their campaigns.
Six states - all in the Northeast except Iowa - and the District of Columbia allow same sex marriages. In addition, two other states have laws that are not yet in effect and may be subject to referendums
The North Carolina amendment was placed on the ballot after Republicans took over control of the state Legislature after the 2010 elections, a role the GOP hadn't enjoyed for 140 years.
North Carolina is the latest presidential swing state to weigh in on gay marriage. Florida, Virginia and Ohio all have constitutional amendments against gay marriage, and Obama's election-year vagueness on gay marriage has come under fresh scrutiny.
The Associated Press contributed to this report.
