Thailand (known until 1939 as Siam) has never been heavily populated. In 1668 an Indian king was reported to have commented somewhat disparagingly to a Siamese visitor that “the King of Golconda is a king of men, while your king is only a king of the forests and the mosquitoes!” By the 1800s Thailand’s population remained low at 2,000,000, and by 1950 it had risen to only 20,041,628. By 2001 the total population had increased to 61,797,751, giving a population density of 120 persons per sq km (312 per sq mi), still one of the lowest in Asia.

The diversity of ethnic, linguistic, and religious groups in Thailand is characteristic of most nations of Southeast Asia, where shifting political boundaries have done little to impede the centuries-long migrations of people. Thailand's central position on the mainland has made it a crossroads for these population movements. As a result, speakers of all four major language families are represented in the country.