Prem Tinsulanonda (Thai: เปรม ติณสูลานนท์, RTGS: Prem Tinnasulanon, pronounced [prēːm tīn.nā.sǔː.lāː.nōn]; 26 August 1920 – 26 May 2019) was a Thai military officer, politician, and statesman who served as the 16th prime minister of Thailand from 1980 to 1988.

During Prem's tenure as prime minister, he was credited with ending a communist insurgency and presiding over accelerating economic growth. As president of the Privy Council, he served as Regent of Thailand from the death of King Bhumibol Adulyadej on 13 October 2016 until 1 December 2016, when Vajiralongkorn was proclaimed King. At the age of 98, Prem was the longest-living Thai prime minister. He is also the oldest regent of any country, surpassing Bavarian Prince Regent Luitpold's record, when he became the regent for king Rama X.

During the Thai political crisis of the mid-2000s, he was accused by deposed prime minister Thaksin Shinawatra and his supporters of masterminding the 2006 coup, as well as in the appointment of the post-coup legislature and interim government of Surayud Chulanont. The military junta that ousted Thaksin denied that Prem had any important political role. Prem, as the President of the Privy Council, promoted King Bhumibol's ideologies and royal projects, though he sometimes represented himself as being the voice of the king. He urged Thai society to follow the king's advice and himself founded several welfare projects related to education, drug suppression, poverty, and national unity. A southerner, Prem had also dealt personally with trying to resolve the South Thailand insurgency. He supported societal corporatism.

Born in Songkhla Province in south Thailand, Prem was the son of Luang Winittantagum (Bueng Tinsulanonda) and Odd Tinsulanonda and had seven siblings. His father was the warden of Songkhla prison, and Prem jokingly claimed to have spent most of his childhood in prison. Prem attended Maha Vajiravudh Secondary School in Songkhla, followed by Suankularb Wittayalai School in Bangkok. He entered the Royal Thai Army Academy (now Chulachomklao Royal Military Academy) in 1941. A distinguished army officer, he entered politics in 1959, as a member of the Constitution Drafting Committee. From 1968 to 1971 he was a Senator, in 1972–73 was a Member of Parliament, and in 1976 was appointed to the Advisory Council of Prime Minister Thanin Kraivichien. Under Prime Minister Kriangsak Chamanan, he was Deputy Interior Minister in 1977–78 and Minister of Defence from 1979 to 1986.

General Prem is known for initiating the negotiations with the members of the Communist Party of Thailand. Consequently, an amnesty was declared and many communist members — including former student protesters — returned home. This helped end the fighting between the government and communist guerrillas in the 1980s.

After Kriangsak retired in 1980, Prem was chosen as Prime Minister. Prem led three administrations and often shifted coalition partners.

From 1–3 April 1981, a group of army colonels known as "The Young Turks" launched a coup attempt in Bangkok. Prem escorted the King and Queen to Nakhon Ratchasima, and began negotiating with the coup leaders. On 3 April, major leaders agreed to end their "April Fool's Day" coup attempt. Some were allowed to take refuge abroad.

Another coup attempt took place on 9 September 1985. Its leaders had been involved in the previous coup four years earlier. The attempt became violent when rebel soldiers fired at the government's information centres, killing an Australian journalist and his American sound man. The coup attempt was supported by Ekkayuth Anchanbutr, a businessman who had fled the country after Prem's government issued new legislation against financial crime. By late afternoon of the same day, the rebels surrendered to the government. Most of its leaders, including Ekayuth, fled abroad.