Xinhua News Agency, quoting Cambodian Deputy Prime Minister Nhiek Bunchhay, said Sihanouk, a father figure in his country who had been battling a number of ailments, died of natural causes.
"Our former king died at 2 a.m. early Monday in Beijing due to natural cause," he was quoted as saying. "This is a great loss for Cambodia. We feel very sad."
Sihanouk, who helped free Cambodia from French rule in 1953, had led the country through its most turbulent times until his abdication in favor of his son in 2004.
The country's King Norodom Sihamoni, the oldest son of the departed ruler, was to fly to Beijing to take his father's body back to Cambodia for a traditional funeral, Xinhua reported.
Chinese doctors had been treating him for years for various forms of cancer, diabetes and hypertension, the report said.
It was from Beijing that the departed monarch, citing his health problems, had announced his abdication.
Sihanouk, the eldest son of King Norodom Suramarit and Queen Kossamak, studied in French schools in Saigon and in Paris. He was made king in 1941 when he was only 18 years old by France's Vichy government, the BBC reported.
However, immediately after the war, Sihanouk began his campaign to win independence for Cambodia.
The New York Times reported Sihanouk was brought down during a coup following the secret bombing of Cambodia in 1969 by the Nixon administration.
That led him to make a deal with the Khmer Rouge, only to see the brutal regime take over his country in 1975.
The Times said under criticism for his shifts in allegiances, Sihanouk defended himself by saying his only course in politics was the "defense of the independence, the territorial integrity and the dignity of my country and my people."
Sihanouk returned in 1993 as monarch and head of state following a U.N. accord.