"We in New Hampshire have had a long and proud tradition taking the lead and opposing discrimination," Lynch said. "Today that tradition continues."
Couples who enter civil unions will have the same rights, responsibilities and obligations as married couples. Same-sex unions from other states also would be recognized if they were legal in the state where they were performed.
Legislators who gathered for the bill signing packed the governor's chambers and overflowed into an adjoining sitting room. They snapped photos and burst into applause as he signed it.
"I've listened and I've heard all the arguments," said Lynch, a Democrat. "I do not believe that this bill threatens marriage. I believe that this is a matter of conscience and fairness."
Episcopal Bishop V. Gene Robinson also attended the bill signing. He and his longtime partner plan to take advantage of civil unions.
Massachusetts alone among the U.S. states allows gay marriage. Connecticut, Vermont, New Jersey, Maine, California 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.
New Hampshire is the first state to embrace same-sex unions without a court order or the threat of one. Connecticut adopted civil unions two years ago while a lawsuit was pending.
The bill's success was a turnabout from two years ago, when a study panel recommended against any meaningful consideration of civil unions and endorsed a constitutional amendment to limit marriage to unions between a man and a woman.
But Democrats won control of the Legislature last fall for the first time in more than a century. Civil unions passed both Houses largely along party lines, and Lynch promised to sign it.