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 today afternoon.
Vajiralongkorn surprised many earlier this year by ordering rewrites to parts of the charter that deal with his powers.
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But in a sign of the opacity surrounding all things royal in Thailand, authorities have yet to release the wording of those new sections.
Thailand has stumbled through more than a decade of political instability that has hampered growth in what was once one of the region's fastest growing economies.
In a period dubbed "the lost decade" Thais witnessed repeated rounds of deadly protests, a string of short-lived governments and two military coups.
Analysts say the latest constitution harks back to the Cold War-era when Thailand's elected 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.
The kingdom's political rift broadly pits rural and poor supporters of ousted premiers Thaksin and Yingluck Shinawatra versus a military-backed Bangkok middle class and business elite.
Shinawatra-linked parties have won every election since 2001. Their opponents accuse them of corruption and damaging populist policies.