Retired General Surayud Chulanont has been sworn in as Thailand's interim prime minister in a brief ceremony in the capital, Bangkok.
His government would focus on "people's happiness" above economic growth, he told reporters afterwards.
He was sworn in by coup leader General Sondhi Boonyaratkalin.
The military also unveiled a short-term constitution under which they will maintain substantial powers until elections promised for October 2007.
Army officers took over the country after a bloodless coup on 19 September, ousting Prime Minister Thaksin Shinawatra.
The swearing-in ceremony for Gen Surayud, 62, came hours after the announcement the former army commander would be the new interim leader.
"The king has appointed him to administer the country from now on," Gen Sondhi said during the ceremony at Government House.
Gen Surayud has previously spoken out against military involvement in politics, but he told reporters he felt he had to take the job "because of the necessity to address the country's problems".
He said he would try to tackle political divisions and end an Islamic insurgency in the country's south.
"I will put these two issues at the top of my agenda," he said. "I will make a great effort to solve these two problems within one year."
Gen Surayud wore a white uniform for the ceremony and prostrated himself in front of a giant portrait of King Bhumibol as a sign of respect. He stood alongside six of the leading officers behind the coup.
A new cabinet is due to be chosen this week.
Gen Sondhi was quoted on news agency Reuters saying he visited Gen Surayud on 28 September "and spent half an hour convincing him to take the job while the country is in crisis".
Gen Surayud is an army veteran, and one of the few senior Thai figures who is respected by military and civilian leaders alike.
By choosing him as prime minister, the military may hope to allay international fears.
The coup leaders say the military had to overthrow the former government to stop systematic corruption.
General Chulanont has been sworn in a brief ceremony
Mr Thaksin, who came to power in 2001, had proved a controversial and divisive prime minister.
Poorer voters had liked his offers of cheap medical care and debt relief, his nationalist platform and his contempt for the "Bangkok elite" in politics. He also won respect for his handling of the aftermath of the 2004 tsunami and policies which led to an economic boom.
Mounting protests earlier this year undermined his position.
But it was his easy victory in April's general election - subsequently declared invalid - that may have prompted the generals to act.