The Senate passed the final reading of a marriage equality law on Tuesday, paving the way for the country to become the third territory in Asia to recognise same-sex couples after Nepal and Taiwan.
The Senate voted 130-4 to pass the law with 18 abstentions. It will be sent to the palace for royal approval. The law will come into force 120 days after it is published in the Royal Gazette.
LGBTQ+ advocates called the move a "monumental step forward" as it would make Thailand the first country in Southeast Asia to enact marriage equality legislation.
Thailand is already known for its vibrant LGBTQ+ culture and tolerance, making it a popular destination for tourists.
"This would underscore Thailand's leadership in the region in promoting human rights and gender equality," the Civil Society Commission of marriage equality, activists and LGBTI+ couples said.
The bill is the culmination of more than a decade of effort from activists and politicians, after previous drafts did not reach the parliament.
At the start of June, thousands of LGBTQ+ revellers and activists held a parade through the streets of Bangkok and were joined by Prime Minister Srettha Thavisin, who wore a rainbow shirt to celebrate Pride Month.
The prime minister will open Government House to celebrate the passing of the bill. Its supporters will hold a Pride parade from Parliament to the office of the prime minister.
Mr Srettha will join them online as he is taking several days off after being diagnosed with Covid-19.