Thai lawmakers on Thursday gave initial approval to legalising same-sex unions, moving the kingdom a step closer to marriage equality.

The legislation must pass through several more stages before becoming law, but if it is enacted, Thailand would be the first country in Southeast Asia to recognise same-sex marriage.

The lower house of parliament voted through a first reading of plans to amend references to "men", "women", "husbands" and "wives" in the marriage law to gender-neutral terms.

The change would allow all Thais to marry without restrictions on sex or gender, as well as conferring adoption and inheritance rights.

"This law would reflect the government's agenda to improve human rights... so that everyone will have the right to build a family without any limitation," Deputy Prime Minister Somsak Thepsutin told lawmakers.

During the debate, some MPs adorned their seats and desks with rainbow flags.

Lawmakers backed the plans by 360 votes to 10, and the proposals will now go before a committee to be scrutinised in detail before a second vote.

Prime Minister Srettha Thavisin welcomed the vote on social media platform X, formerly Twitter.

"We are finally on the road to bridging the gap to equal rights for all today!" he posted.

"Congratulations to the LGBTQIA+ community for the Same-Sex Marriage Bill passing its first reading. May love finally triumph."

Similar proposals were given initial approval by parliament in 2022, but the law progressed no further because the house was dissolved for a general election. (AFP)