Ko Yao (Thai: เกาะยาว, pronounced [kɔ̀ʔ jāːw]) is a district (amphoe) in Phang Nga province in Thailand's south.

Cave paintings discovered on Ko Yao Island, dating back over 2,000 years, provide evidence of historical influences on the communities in the southern mainland provinces of Thailand.

[1][2] The Mon people, part of the Khmer ethnolinguistic group, established settlements in peninsular Thailand, including maritime states like Ligor (Nakhon Si Thammarat).

Over the centuries, they intermingled with migrants from northern and southern regions of Malaysia and Thailand through commercial exchange and political interactions.

This integration has resulted in the Mon ethnic lineage being a prominent component of the demographic makeup in southern Thailand, including the population of Ko Yao.