The move illustrates the close military-royal alliance that has defined Thai politics for the last five decades, an era that has seen brief flirtations with democracy punctuated by multiple palace-endorsed coups.
The most recent coup was in 2014, when generals ousted an elected government. Analysts believe it was staged to control the looming royal succession as the former king's health declined.
The deeply revered King Bhumibol Adulyadej died in October aged 88 after a seven-decade reign, leaving the politically turbulent country bereft of a unifying figure.
The royal succession is a highly sensitive topic in Thailand, where criticism of the monarchy is banned, and has thrust the kingdom into a new era of uncertainty.
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Today's statement on television announced that Vajiralongkorn has trimmed the size of his Privy Council from 16 to 11, keeping eight members appointed by his father and elevating three new ones.
The new appointees are three top generals associated with the junta. Two are members of the military government's current cabinet, Justice Minister General Paiboon Koomchaya and Education Minister General Dapong Ratansuwan.
They join three ex-generals who keep their positions, giving the armed forces a majority on the council.
The military has long styled itself as the defender of the monarchy, almost always staging coups in its name.
Privy councillors have been appointed interim prime ministers after some previous coups, and wield significant political influence behind the scenes.
While the constitutional monarchy has limited formal power, it controls one of the world's richest crowns and attracts the loyalty of much of the business and military elite.
The junta has ramped up use of the law since its power grab, throwing scores of people behind bars -- sometimes for decades.