last absolute king of Siam (1925–35), under whose rule the Thai revolution of 1932 (Promoters Revolution) instituted the constitutional monarchy. Prajadhipok never expected to succeed to the throne. He was the 32nd and last son of King Chulalongkorn, the youngest of five sons by Queen Saowabha.

When King Vajiravudh died in 1925, Prajadhipok had been a likely heir to the throne for less than a year and the certain heir for only two days. He had been prepared for a military career at Eton College and the Royal Military Academy at Woolwich, Eng. Though personally convinced of the necessity of moving toward democratic political reforms, he allowed himself to be restrained by senior members of the royal family, and his inactivity brought on the Thai revolution of 1932, which bloodlessly ended the absolute monarchy. Prajadhipok welcomed the opportunity to reign as a constitutional monarch but was repulsed by growing military rule and abdicated on March 2, 1935. He died in exile in England.