There is evidence of continuous human habitation in present-day Thailand since 20.000 years ago.

An excerpt from the article 31 facts about Thailand

The earliest evidence of rice cultivation dates back to 2000 BC. The archaeological site of Ben Chiang in northeastern Thailand is currently the oldest known center of copper and bronze production in Southeast Asia.

The first and most powerful kingdom in Southeast Asia in the 2nd century BC was the kingdom of Funan. From the time of this kingdom until the rise of the Khmer Empire with its capital at Angkor in the 9th century, the Indochinese Peninsula was strongly influenced by Indian culture and religion.