The Yai River is a river of

Thailand Thailand, officially the Kingdom of Thailand and historically known as Siam (the official name until 1939), is a country in Southeast Asia on the Mainland Southeast Asia, Indochinese Peninsula. With a population of almost 66 million, it spa ...

. It is a tributary of the

Pa Sak River The Pa Sak River (, , Pronunciation is a river in central Thailand. The river originates in the Phetchabun Mountains, Dan Sai District, Loei Province, and passes through Phetchabun Province as the backbone of the province. It then passes throu ...

, part of the

Chao Phraya River The Chao Phraya River is the major river in Thailand, with its low alluvial plain forming the centre of the country. It flows through Bangkok and then into the Gulf of Thailand. Etymology Written evidence of the river being referred to by the ...

basin.

See also