"The king has approved the interim constitution," the military said in a statement read on national television at precisely 9.29am local time on Sunday, considered an auspicious time in Thai culture.
Bhumibol Adulyadej's endorsement came after the leaders of the military coup, now grouped in the Council for Democratic Reform (CDR), promised to hand power to a civilian government before their self-imposed two-week deadline expires on October 4.
The interim constitution guarantees basic human rights, installs a 36-member cabinet which will be able to pass laws and creates an assembly of 2,000 representatives to select a panel to write a new constitution, the military said.
The 39-article charter transforms the CDR into the Council for National Security, which will oversee national security.
It will also have the power to approve - and veto - the appointment of people to key government positions including the national assembly.
The military abolished a 1997 constitution after seizing power from Thaksin Shinawatra, the prime minister, in a bloodless coup on September 19.
They have said that the previous constitution had too many "loopholes" which allowed Thaksin and his supporters to profit from widespread corruption.
Colonel Akara Thiprot, a spokesman for the junta, said that the appointment of a new prime minister would be announced later on Sunday.
It is widely believed that this will be a former army commander, Surayud Chulanont, a retired general who has served as a close adviser to the popular constitutional monarch.