The bill sailed through on 399 to 10 votes in the lower house, although it must still be approved by the Senate before it is endorsed by the king, and then published in the Royal Gazette.

After the result, one representative brought a rainbow flag into the chamber.

"Today society has proved to us that they care about LGBT rights," Tunyawaj Kamolwongwat, an MP with the progressive Move Forward Party which has pushed for LGBTQ+ rights, told AFP.

"Now we finally we will have the same rights as others."

Across Asia only Taiwan and Nepal recognise same-sex marriage, with India's highest court deferring the decision to parliament in October.

And while the kingdom enjoys a welcoming reputation for the international LGBTQ+ community, Thai activists have struggled for decades against conservative attitudes and values.

The proposal will change references to "men", "women", "husbands" and "wives" in the marriage law to gender-neutral terms.

It also means LGBTQ+ couples will be able to enjoy inheritance and adoption rights in the kingdom for the first time.

- 'A huge step' -

"It's a huge step for our country -- it is the first in Southeast Asia," Mookdapa Yangyuenpradorn, a spokesperson with activist group Fortify Rights, told AFP.

Mookdapa expressed hope that the later stages of the bill would progress smoothly, so that her country "will be on par with the international level in terms of LGBT rights".

The vote follows a decision by Prime Minister Srettha Thavisin's cabinet last year that gave the go-ahead for the parliament debate.

The prime minister has been vocal in his support for the LGBTQ+ community, making the policy a signature issue and telling reporters last year that the change would strengthen family structures.

While Thailand has a reputation for tolerance, much of the Buddhist-majority country remains conservative and the LGBTQ community, while highly visible, still faces barriers and discrimination.

Activists have been pushing for same-sex marriage rights for more than a decade, but in a kingdom where politics is regularly upended by coups and mass street protests, the advocacy did not get far.

In 2022 Thai lawmakers gave initial approval to two bills that would allow same-sex marriages and two others that would permit civil partnerships.

But the legislation was dropped when parliament was dissolved to pave the way for a general election held last year.