The first same-sex marriages will take place in the US state of Hawaii on Monday.
Governor Neil Abercrombie signed the law into effect last month, after calling a special legislative session to get the legislation approved.
Couples will begin to marry from 12.01am on Monday, immediately as the law comes into play. Others will need to wait until 8am, when the state Health Department opens.
“Finally, today now all those who have been invisible will be visible to themselves and the whole world,” Mr Abercrombie said on signing the bill into law.
Earlier, US President Barack Obama has applauded his birthplace Hawaii for becoming the 15th US state to legalise same-sex marriage, saying “I have always been proud to have been born in Hawaii, and today’s vote makes me even prouder.”
The President issued his statement immediately after the state Senate gave its final approval to the bill.
He said: “With today’s vote, Hawaii joins a growing number of states that recognise that our gay and lesbian brothers and sisters should be treated fairly and equally under the law.
“Whenever freedom and equality are affirmed, our country becomes stronger. By giving loving gay and lesbian couples the right to marry if they choose, Hawaii exemplifies the values we hold dear as a nation.”