New Hampshire has "a long and proud tradition taking the lead in opposing discrimination," Lynch said. "Today that tradition continues."
Same-sex unions from other states also will be recognized if they were legal in the state where they were performed. The bill passed last month in both chambers of the Legislature, which Democrats control.
California, Connecticut, Vermont, New Jersey, Maine and Washington allow either civil unions or domestic partnerships, and Oregon will join the list with New Hampshire in January. Hawaii extends certain spousal rights to same-sex couples and cohabiting heterosexual pairs. Massachusetts is the only state that allows same-sex marriage.