Mass migration of Tai peoples from China (Guangxi) to Mainland Southeast Asia and Northern Thailand occurred between the 8th-10th century. The Mainland region was ruled by the Khmer Empire since 800 CE. The Thai established their own kingdoms: the semi-legendary Singhanavati Kingdom (691 BCE–638 CE) evolved into the Ngoenyang Kingdom (638–1292). In 1220, the Khmer controlled Sukhothai was conquered by the Thais and made the capital of the Sukhothai Kingdom. By 1220, the long declining Khmer Empire was mostly overrun by Thais. By the 13th century, the Sukhothai Kingdom (1238–1438) had replaced the Mon kingdoms in Central Thailand. During the reign of King Ramkhamhaeng the Thai script was created in 1283, the arts flourished, Thai institutions were developed, and people called themselves "Thai" as freed people from foreign rule.

In 1351, the Suphannabhum city-state (modern-day Suphan Buri), and Lavo city-state (modern-day Lop Buri) merged into the Ayutthaya Kingdom which was founded near the Chao Phraya River. Duarte Fernandes was the first European to arrive in Ayutthaya in 1511. Siam became the popular international name for Thailand.

There were 26 Burmese-Siamese wars from the 16th to the early 19th century. The Ayuthaya Kingdom collapsed when the capital Ayutthaya city was sacked during Burmese–Siamese War (1765–1767). General Taksin expunged the Burmese, reunified the 5 warring regional states and established the shortlived Thonburi Kingdom in 1767. Taksin was disposed by the Thonburi military commander Chao Phraya Chakri who subsequently founded the Rattanakosin Kingdom (1782–1932). At its greatest extent in 1805-1812 the Rattanakosin Kingdom consisted of 25 polities including modern-day Cambodia, Laos, northern Malaysia and eastern Burma.

King Mongkut (Rama IV) embraced Western innovations and initiated the modernization of Thailand. During European colonization of Southeast Asia (1511-1984) only Thailand remained independent. This was due to multiple factors: the centralizing and modernization reforms enacted by King Chulalongkorn, a political policy which balanced British and French colonial interests, King Rama V made diplomatic visits to Europe in 1897 and 1907, large territorial concessions to French Indochina, and the French and British maintained Siam as a buffer state to avoid conflicts between their colonies. An 1874 edict was issued for reforms which abolished slavery in 1905.

Siam became an ally of the United Kingdom in 1917 and joined the Allies of World War I. The Siamese revolution of 1932 ended centuries of absolute monarchy. The government changed to a constitutional monarchy with King Prajadhipok (Rama VII). The country name was formally changed to Thailand which means “Land of the Free” in 1939. The Franco-Thai War in 1940-1941 resulted in the Thai annexation of disputed territories in Cambodia and Laos of French Indochina. During World War II, Thailand was initially neutral, but joined the Axis after the Japanese invasion of Thailand on 8 December 1941. Thailand annexed disputed territories in Burma (Saharat Thai Doem), and Malaysia (Sirat Malai). After World War II these territories were ceded in return for admission to the United Nations, dropping all wartime claims and U.S. aid.

During the 70-year reign of King Bhumibol Adulyadej, Thailand had 10 coups with military governments and 17 constitutions. During the Vietnam War in 1962, Thailand permitted the United States to use bases and Thai troops fought in South Vietnam. In 1965, the CPT began the Communist insurgency which was won by the Thai Government in 1973.

Thailand was a founding member of ASEAN in 1967. The 1970s peasant revolts in Thailand to reduce the debt of farmers and for fair rice prices lead to the enactment of the Land Rent Control Act (LRCA) in Dec 1974. Thailand had unprecedented economic growth from 1993 to 1997. The 1997 Asian financial crisis caused many bankruptcies and unemployment. However, there was a quick recovery in 1998-1999. In 2000, candidates were democratically elected for the Senate for the first time. The world's largest parliamentary building; the Sappaya-Sapasathan was completed on 1 May 2021.

The name Siam (Thai: สยาม RTGS: Sayam) may have originated from Pali (suvaṇṇabhūmi, "land of gold"), Sanskrit श्याम (śyāma, "dark"), or Mon ရာမည (rhmañña, "stranger"), with likely the same root as Shan and Ahom. The Thai country name has mostly been Mueang Thai. The country's designation as Siam by Westerners likely came from the Portuguese. Portuguese chronicles noted that Borommatrailokkanat, king of Ayutthaya, sent an expedition to the Malacca Sultanate, at the southern tip of the Malay Peninsula, in 1455. Following their conquest of Malacca in 1511, the Portuguese sent a diplomatic mission to the Ayutthaya Kingdom. The explorer Duarte Fernandes was the first European to arrive in Ayutthaya in 1511. The Mandarin name for Siam is Xiān luó (Chinese: 暹罗). A century later, on 15 August 1612, The Globe, an East India Company merchantman bearing a letter from King James I, arrived in "the Road of Syam". "By the end of the 19th century, Siam had become so enshrined in geographical nomenclature that it was believed that by this name and no other would it continue to be known and styled." The country was renamed to Thailand (Thai: ประเทศไทย RTGS: Prathet Thai) which means “Land of the Free” in 1939. Thailand has been nicknamed the "Emerald Kingdom", the "Land of the White Elephant", the "Rice bowl of Asia", and the "Land of Smiles". The elephant is a Thai national symbol and treasure. Thailand's geographic shape resembles an elephant's head with a long trunk.