The military says the charter will curb unrest in the politically-split kingdom and keep out corrupt lawmakers.
The military says the charter -- Thailand's twentieth since 1932 -- will curb unrest in the politically-split kingdom and keep out corrupt lawmakers.
But opponents say the new document means any polls, whose date keeps slipping, will only offer Thais a form of neutered democracy with a fully appointed senate and tough controls on elected politicians.
King Maha Vajiralongkorn, who ascended the throne following the October death of his widely revered father Bhumibol Adulyadej, signed the document in a televised ceremony in Bangkok on Thursday afternoon.
In a solemn and ritual-laden ceremony, the 64-year-old king inked three copies of the document, which were then stamped by white-clad officials and placed on gilded pillars before the monarch.
An official then read the charter's preamble.
"This constitution aims to fully rid the country of corruption, abuses of power and prevent corrupt politicians from taking office," the official read to an audience of political grandees, including junta chief Prayut Chan-O-Cha, and ambassadors in the ornate throne hall.
Vajiralongkorn surprised many earlier this year by ordering rewrites to parts of the constitution that deal with his powers -- a move that delayed its promulgation and the date for elections.
But in a sign of the opacity surrounding all things royal in Thailand, a country with a strictly enforced lese majeste law, authorities have yet to release the wording of those new sections.
The charter received popular approval in a referendum last August, although the junta barred campaigning against it.
The generals have severely curbed free speech since their power grab by outlawing all political activities, muzzling the press and detaining scores of critics.
Democracy has never fully taken in root in Thailand, where the army has launched more than a dozen coups since the end of absolute monarchy in 1932.
Over the past 10 years, an especially turbulent period dubbed "the lost decade," Thais have witnessed repeated rounds of deadly protests, a string of short-lived governments and two military coups that deposed elected leaders.
Analysts say the latest constitution harks back to the Cold War era when Thailand's lawmakers were often kept in check by unelected bodies and committees in what many called "guided democracy".
Thitinan Pongsudhirak, a politics expert at Chulalongkorn University, said the document is a far cry from Thailand's most liberal charter, the 1997 so-called "People's Constitution".
"The new charter reverses progress on people's representation that culminated with the 1997 constitution," he told AFP.
"Now appointments are back en masse at the expense of elected representatives."
Critics say the junta is determined to block a political comeback from ousted premiers Thaksin and Yingluck Shinawatra, whose parties have won every poll since 2001.
Their billionaire clan is popular among Thailand's rural and urban poor but hated by Bangkok's military-backed elite, who accuse the family of corruption, nepotism and damaging populist policies.
In addition to an appointed upper house, the new constitution bolsters the powers of the country's interventionist constitutional court and makes it easier to impeach a civilian leader.
With royal approval secured, the junta's drafting committee is now tasked with drawing up organic laws that critics fear could further hem-in political parties, who have been barred from organising since the coup.
The junta has said elections will be held in mid-2018 but officials have shied from providing a firm date.
They have also vowed to draw up a "20-year plan" for the country that any future government will be bound to follow.
This week ousted Prime Minister Yingluck Shinawatra, who is entangled in a negligence trial since the coup that could see her jailed, said she hoped the charter would speed the kingdom's return to democracy.