The royal birthday celebration for the king, who ascended to the throne in 2004 after the abdication of his father, Norodom Sihanouk, began Tuesday and will continue until Thursday.
The king traditionally conducts Buddhist rites at his palace in the capital, but had chosen to fly by helicopter to an area close to northern Preah Vihear temple, perched on the Thai border, officials said.
He was born in 1953.
Sihamoni's visit will coincide with the opening of a Thai-built road in the south-western province of Koh Kong, to be attended by both Cambodian Prime Minister Hun Sen and Thai Foreign Minister Noppodon Pattama, according to ministerial spokesman Phay Siphan.
Siphan declined to speculate on whether the pair would discuss controversial border issues during the ceremony.
Cambodia is also mulling the idea of a national holiday on June 15 to mark the 1962 anniversary of the International Court in The Hague deciding the temple, which is sacred to both Cambodians and Thais, lay within Cambodian territory.
Preah Vihear, known as Khao Phra Viharn in Thai, is technically in Cambodian territory, but only easily accessible from the Thai side.
Thailand has said it will not dispute Cambodia's proposal to nominate the temple as a World Heritage site this year, but border demarcation, both around Preah Vihear and in disputed coastal waters, remains a sensitive issue for both nations.
Potential oil, gas and mineral deposits remain in the balance depending on the results of negotiations.
Cambodian Information Minister Khieu Kanharith has urged local Cambodian media to remain calm and not inflame debate over the temple with nationalistic rhetoric.
In 2003, an angry Cambodian mob burned the Thai embassy and Thai businesses over a false rumour that a Thai soap star had claimed the iconic Angkor Wat temple was not Cambodian.//dpa