Late on Friday evening, New York became the sixth US state to legalise gay marriage.
After days of negotiation over protections for religious groups, senators voted 33-29 to pass the bill.
Governor Andrew Cuomo has already signed it into law although thirty days must pass before the first ceremonies can take place.
New York is America’s third most populous state and the breakthrough doubles the number of gay Americans who can marry in their own state.
As New York has no residency requirement, gay couples from other states are expected to take advantage of the law.
The victory is a stunning success for gay rights campaigners who need nationwide momentum.
Evan Wolfson of Freedom to Marry New York described it as “without a doubt a watershed moment for our country and our campaign”.
After signing the bill, Mr Cuomo told reporters: “I think this vote today will send a message across the country.
“This is the direction to go, and the time to do it is now, and it is achievable.”
Last night, hundreds of gay marriage supporters flocked to the Stonewall Inn, the iconic birthplace of the gay rights movement, to celebrate the vote.
Religious protections in the bill allow any faith group to refuse to provide services for gay marriages and this refusal “shall not create any civil claim or cause of action or result in any state or local government action to penalise, withhold benefits, or discriminate against” the organisations.
Four Republicans voted for the bill and the anti-gay National Organization for Marriage said it would redouble efforts to have them voted out of office in 2012.
The British and Scottish governments are to consult on the introduction of full gay marriage. However, civil partnerships, as recognised across the whole of the United Kingdom offer nationwide rights including for taxation. Gay marriages that are conducted in New York state will not be recognised across the whole of the USA and do not confer federal tax benefits that are available to straight married couples.