The late king was widely seen as a pillar of stability during seven decades of political turmoil in Thailand.
His son is yet to enjoy the same level of popularity as his father, and spends much of his time abroad.
The crown prince had been expected to become king the day after his father died but Prime Minister Prayuth Chan-ocha at the time said that he had asked to delay the official proclamation so he could mourn.
The ascension will end a period of uncertainty during which Privy Council President Prem Tinsulanonda, a 96-year-old former prime minister, has been serving as regent.
Thailand's monarchy is protected from criticism by tough lese-majeste laws, which restrict media discussion about the royal family's role, including in the international press.
Most ordinary Thais know only a few details about who the crown prince is and how he lives his life.
In recent years he has tried to improve his profile - important because the king is traditionally seen as a guiding force in Thai politics, which is highly polarised.
A formal coronation will not happen until after King Bhumibol's cremation, expected next year.
Bhumibol's own coronation took place four years after his brother King Ananda Mahidol died of gunshot wounds in mysterious circumstances.