Thailand on Tuesday became the first country in Southeast Asia to legalise same-sex marriage, in a historic parliamentary vote hailed as a "victory" by campaigners.

The upper house Senate gave final approval -- by 130 votes to four, with 18 abstentions -- to changes to the marriage law allowing same-sex couples to tie the knot.

The new legislation will now go to King Maha Vajiralongkorn for royal assent and come into force 120 days after publication in the official Royal Gazette.

Thailand will become only the third place in Asia where same-sex couples can get hitched, after Taiwan and Nepal, and activists are hoping the first weddings could be celebrated as early as October.

"We are very proud of everyone involved in this historic moment. You have helped to bring about a massive change," Plaifah Kyoka Shodladd, an LGBTQ activist and member of the committee that scrutinised the law, told senators after the vote.

"Today love wins over prejudice." 

Ahead of the vote, Tunyawaj Kamolwongwat, an MP with the progressive Move Forward Party, said the change in the law was "a victory for the people".

The new legislation changes references to "men", "women", "husbands" and "wives" in marriage laws to gender-neutral terms.

It also gives same-sex couples the same rights as heterosexual ones when it comes to adoption and inheritance.

"My partner has one boy and I want to have legal rights to formally adopt him as my child and to have a say about his well-being. This bill will allow it," Kevin Pehthai Thanomkhet, 30, told AFP.

Prime Minister Srettha Thavisin, who has been vocal in his support for the LGBTQ community and the bill, opened his official residence to activists and supporters for a celebration after the vote.

"We have fought a long time because we believe in all equal rights," Srettha wrote on his X account after the vote.

"Today is our day. We celebrate to 'diverse' love, not 'different'. Love is beautiful and powerful."

In central Bangkok, a crowd of activists celebrated the news by watching a drag show and decorating the grounds of the Bangkok Art and Culture Centre with a giant rainbow flag.

"As a Thai person, I am very proud," Korakoch Jeumsanga, a 23-year-old a 23-year-old spectator, who was watching the event, told AFP at the celebration. 

"This bill benefits both straight and gay people. I had goosebumps when the bill passed."

Miles Enriquez-Morales, a tourist from California, said he hoped more countries would follow Thailand's example.

"With all the anti-LGBTQ sentiment in US and Europe, I hope that more countries will follow suit in legalising same-sex marriage in different parts of the world," he said.

Joe Yang, 32, a tourist from Guangzhou in China who attended the event with friends, said the outcome of the vote was "great news" for Thailand.

"This would not happen in China," he said.

"You know given the reputation of Thailand I thought that you guys would have legalised it already. I am happy for Thai people."

Long struggle

Thailand has long enjoyed a reputation for tolerance of the LGBTQ community, and opinion polls reported in local media show overwhelming public support for equal marriage.

More than 30 countries around the world have legalised marriage for all since the Netherlands became the first to celebrate same-sex unions in 2001.

But in Asia only Taiwan and Nepal recognise marriage equality. India came close in October, but the Supreme Court referred the decision back to parliament.

"I am so happy to see how far we have come," said Chotika Hlengpeng, a participant in the Pride march that drew thousands of enthusiasts in Bangkok early in June.

Tuesday's vote is the culmination of years of campaigning and thwarted attempts to pass equal marriage laws.

While the move enjoys popular support, much of Buddhist-majority Thailand still retains traditional and conservative values.

LGBTQ people, while highly visible, say they still face barriers and discrimination in everyday life.

And some activists have criticised the new laws for failing to recognise transgender and non-binary people, who will still not be allowed to change their gender on official identity documents.