It follows two decades of campaigning by activists who hailed the move a "monumental step forward".

The bill - which was supported by an overwhelming majority of politicians in the upper house and grants full legal, financial and medical rights for marriage partners of any gender - just needs the endorsement of King Maha Vajiralongkorn before it becomes law.

The legislation will come into force 120 days after it is published in the royal gazette, meaning the first same-sex weddings could take place in Thailand later this year.

"We are very proud to make history," said Plaifah Kyoka Shodladd, a member of a parliamentary committee on same-sex marriage.

"Today love triumphed prejudice... after fighting for more than 20 years, today we can say that this country has marriage equality."

"Today we celebrate another significant milestone in the journey of our Equal Marriage Bill," Thai Prime Minister Srettha Thavisin said in a post on X.

"We will continue our fight for social rights for all people regardless of their status."

The bill passed its final reading with the approval of 130 of the 152 members of the senate in attendance, with four voting against it and 18 abstaining.

Once the law receives royal approval, Thailand will become the third place in Asia - after Taiwan and Nepal - to allow same-sex marriage.

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Thailand is one of Asia's popular tourist destinations and has a reputation for acceptance and inclusivity.

However, Thai society largely holds conservative values, so the country has struggled for decades to pass a marriage equality law, while members of the LGBT+ community say they face discrimination in everyday life.

The passing of the bill "would underscore Thailand's leadership in the region in promoting human rights and gender equality" - the Civil Society Commission of marriage equality, activists and LGBTQI couples said in a statement.