Tai peoples who originally lived in southwestern China, migrated into mainland Southeast Asia over a period of many centuries. The oldest known mention of their existence in the region by the exonym Siamese is in a 12th century A.D. inscription at the Khmer temple complex of Angkor Wat in Cambodia, which refers to syam, or "dark brown" people. It was believed that Siam derived from the Hindi word shyam, or brown race, with a contemptuous signification. During the reign of Rama III (1824–1851,) a Scottish trader had experimental coins struck in England at the king's behest, Though not adopted for use, the name of the country put on these first coins was Muang Thai, not Siam. Also spelled Siem, Syâm or Syâma, it has been identified with the Sanskrit Śyâma (श्याम, meaning "dark" or "brown"). The names Shan and A-hom seem to be variants of the same word, and Śyâma is possibly not its origin but a learned and artificial distortion.

The country's designation as Siam by Westerners likely came from Portuguese, the first Europeans to give a coherent account of the country. Portuguese chronicles noted that the king of Sukhothai had sent an expedition to Malacca at the southern tip of the Malay Peninsula in 1455. Following their conquest of Malacca in 1511, the Portuguese sent a diplomatic mission to Ayutthaya. A century later, on August 15, 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."