Thailand has a large gay community, but remains largely conservative, although homosexual, transgender and transsexual people play key roles in its entertainment industry.
Thai law does not recognise same-sex unions, which keeps gay couples from taking joint bank loans and medical insurance, besides barring changes to gender categories on national identity papers.
The Constitution Drafting Committee, a group hand-picked by the military to prepare a new constitution after the previous one was scrapped following a May coup, began work this week.
Panel spokesman Kamnoon Sittisamarn said the new measure would ensure all sexual identities were protected under the constitution and treated equally by the law.
"We are putting the words 'third gender' in the constitution because Thai society has advanced," he told Reuters.
"There are not only men and women, we need to protect all sexes. We consider all sexes to be equal."
The panel will send details of the measure to the National Reform Council by April. It will need to be formally approved by the ruling junta, also known as the National Council for Peace and Order.
In 2012, a group of lawmakers and LGBT activists formed a committee to draft legislation recognising same-sex couples that would, among other things, enable them to marry.
But discussion of the draft law was put on ice while Thailand struggled with political protests in 2013 and 2014.
Gay rights activists welcomed the decision to include the term "third gender" in the new constitution.
"It would treat all citizens equally and help to protect from discrimination in all areas including ease of doing business and also personal life," said prominent gay activist Natee Teerarojjanapongs.
The army seized power on May 22 to restore order after months of political infighting that killed nearly 30 people. It scrapped a 2007 constitution for an interim charter giving the military sweeping powers, and protection for the coupmakers.