The governor of Hawaii, Neil Abercrombie, signed a civil unions bill into law yesterday.
The measure was approved by the Senate last week and will give gay couples almost all of the rights of marriage.
Signing the bill at a televised ceremony, Mr Abercrombie said: “This signing today of this measure says to all the world that they are welcome, that everyone is a brother or a sister in paradise. The legalisation of civic unions in Hawaii represents in my mind, equal rights.”
Around 150 gay activists attended the event, cheering as the governor as he declared the bill signed.
The first ceremonies will take place on January 1st 2012 and will make Hawaii the seventh US state to allow civil unions.
Gay rights campaigners are still pushing for marriage equality, which is legal in five states – Massachusetts, Connecticut, Iowa, New Hampshire, Vermont and Washington DC.