Hawaii legalized same-sex civil unions, as its governor signed into law a bill giving gay couples the same rights as heterosexual married partners.
Gay rights campaigners on Wednesday welcomed the signature by Governor Neil Abercrombie, following approval by lawmakers last week, although they said it did not allow gay couples to formally marry.
"This bill has been a long time coming for committed couples in Hawaii who have been denied the basic right to take care of their families," said Laurie Temple, a lawyer for the American Civil Liberties Union (ACLU) of Hawaii.
"While we continue to work to achieve the freedom to marry for all couples, we commend the legislature and Governor Abercrombie for taking a stand against baseless discrimination by passing this bill."
Abercrombie had had 10 days to sign the bill into law after it was passed by Hawaii's senate by 18 votes to five last Wednesday. The law will come into force on January 1, 2012.
"This signing today of this measure says to all the world that they are welcome, that everyone is a brother and a sister here in paradise," the governor told reporters.
"The legalization of civil unions in Hawaii represents in my mind equal rights for all people. They protect our diversity, they protect our civil rights, they protect who we are here in Hawaii."
Six US states have legalized gay marriage: Connecticut, Iowa, Massachusetts, New Hampshire, Vermont and the capital Washington, while civil unions are legal in others including California, Nevada, Oregon, Washington and New Jersey.
Abercrombie said he was proud that the tourist-friendly Pacific island state had resolved differences to agree on the legislation.
"Our diversity defines us rather than divides us. It's no secret that there has been controversy. Here in Hawaii, we do not let our diversity divide us, it indeed defines us, and this will define us," he said.
James Esseks, head of the ACLU Lesbian, Gay, Bisexual and Transgender Project, said: "With today's historic action in Hawaii, we are closer to achieving the goal of fairness and dignity for all families.
"Across the nation, we are seeing steps toward providing committed couples with the security of knowing they can take care of each other, and that their relationships are recognized in the eyes of the law."
Hawaii's move came on the same day of another major victory for gay rights advocates, when President Barack Obama announced his administration will no longer defend a law defining marriage as only between a man and a woman.