The Thai king has signed same-sex marriage into law, the official Royal Gazette said on Tuesday, making Thailand the first country in Southeast Asia to recognise marriage equality.

King Maha Vajiralongkorn gave royal assent to the new law, passed by parliament in June, which will take effect in 120 days – meaning the first weddings are expected to take place in January.

Activists hailed a "monumental step" as Thailand becomes only the third place in Asia where same-sex couples can tie the knot.

The law on marriage now uses gender-neutral terms in place of "men", "women", "husbands" and "wives", and also grants adoption and inheritance rights to same-sex couples.

The king's formal approval marks the culmination of years of campaigning and thwarted attempts to pass equal marriage laws.

"The law is a monumental step towards equal rights in Thailand," said Waaddao Chumaporn, an LGBTQ rights advocate.

She plans to organise a mass wedding for more than a thousand LGBTQ couples in Bangkok on January 22, the first day the law takes effect.

"We are all delighted and excited. We've been fighting for our rights for over 10 years, and now it's finally happening," Siritata Ninlapruek, an LGBTQ activist, told AFP, her voice shaking.

Prime Minister Paetongtarn Shinawatra posted congratulations "for everyone's love" on social media platform X.

"Thank you for the support from all sectors. It is a joint fight for everyone," she wrote with the "lovewins" hashtag.

Thailand has long had an international reputation for tolerance of the LGBTQ community, and opinion polls reported in local media have shown overwhelming public support for equal marriage.

However, much of the Buddhist-majority kingdom retains traditional and conservative values and LGBTQ people say they still face barriers and discrimination in everyday life. (AFP)