Bhumibol Adulyadej (Thai: ภูมิพลอดุลยเดช; RTGS: Phumiphon Adunyadet; pronounced [pʰuːmípʰon ʔàdunjádèːt] ( listen); see full title below; born 5 December 1927) is the reigning King of Thailand. He is known as Rama IX. Having reigned since 9 June 1946, he is the world's longest-serving current head of state and the longest-reigning monarch in Thai history.

King Bhumibol is respected and revered by many Thais. The King is legally considered "inviolable", and lèse majesté, that is, offence against the dignity of the monarch may be punished. In 1957, the overthrow of the government was justified with allegations of lèse majesté. Bhumibol invited public criticism in a 2005 speech, resulting in thousands of arrests.

Bhumibol is credited with a social-economic theory of self-sufficiency. Forbes estimated Bhumibol's personal fortune, including property managed by the Crown Property Bureau which is considered national property, to be US$30 billion in 2010, and he has been consistently placed at number one of the magazine's list of "The World's Richest Royals". This estimation includes land ownership which the royal family says it does not own. The Crown Property Bureau spends money on public welfare such as youth development, however it does not pay taxes and its finances are reported only to Bhumibol. Bhumibol himself has made donations to numerous development projects in Thailand, in areas including agriculture, environment, public health, occupational promotion, water resources, communications and public welfare. Commemorations of Bhumibol's contributions to Thailand are ubiquitous in the Thai media.