Princess Sirindhorn of Thailand, the Princess Royal and the Princess Debaratana Rajasuda , formerly Princess Sirindhorn Debaratanasuda Kitivadhanadulsobhak, is the second daughter of King Bhumibol Adulyadej and younger sister of King Vajiralongkorn. Thais commonly refer to her as "Phra Thep", meaning "princess angel". Her title in Thai is the female equivalent of the title once held by her brother, King Maha Vajiralongkorn. The Thai constitution was altered in 1974 to allow for female succession, thus making her eligible for the throne. Having been the eldest female child of the royal family, her position is comparable to a princess royal.

Bhumibol Adulyadej, conferred with the title King Bhumibol the Great, was the ninth king of Thailand from the Chakri dynasty, titled Rama IX, from 1946 until his death in 2016. His reign of 70 years and 126 days is the longest of any Thai monarch, the longest of an independent Asian sovereign and the third longest verified reign of any monarch of a sovereign state in history after Louis XIV and Elizabeth II.