The bill now requires approval from the Senate and endorsement from the king before it becomes law. It had the support of all of Thailand's major parties and was passed by 400 of the 415 lawmakers present, with 10 voting against it.
"We did this for all Thai people to reduce disparity in society and start creating equality," Danuphorn Punnakanta, chairman of the parliamentary committee on the draft bill, told lawmakers ahead of the reading.
"I want to invite you all to make history."
The passing of the bill marks a significant step towards cementing Thailand's position as one of Asia's most liberal societies on lesbian, gay, bisexual and transgender issues, with openness and free-wheeling attitudes coexisting with traditional, conservative Buddhist values.
Thailand has long been a draw for same-sex couples, with a vibrant LGBT social scene for locals and expatriates, and targeted campaigns to attract LGBT travellers.
The bill could take effect within 120 days of royal approval. Thailand would follow Taiwan and Nepal in becoming the first places in Asia to legalise same-sex unions.
The legislation has been more than a decade in the making, with delays due to political upheaval and disagreement on what approaches to take and what should be included in the bill.
The Constitutional Court had in 2020 ruled Thailand's current marriage law, which only recognises heterosexual couples, was constitutional, recommending legislation be expanded to ensure rights of other genders.
Parliament in December approved four different draft bills on same-sex marriage in the first reading and tasked a committee to consolidate those into a single draft.
