thumb|float|300px|Pa Sak River floating east of Ayuttaya's old city.
The Pa Sak River () is a river in central Thailand. The river originates in Dan Sai District, Loei Province, passes through Phetchabun Province as the backbone of the Province. It then passes through the eastern part of Lopburi Province and Saraburi Province, until it joins together with the Lopburi River at the northeast of the Ayutthaya Island, before it runs into the Chao Phraya River in the southeast of Ayutthaya island around Phet Fortress area. It has a length of 513 km and drains a watershed of 16,291 km². The annual discharge is 2.4 km³.

The valley of the Pa Sak through the Phetchabun mountains is the main part of Phetchabun Province. As the watershed of the river is rather narrow, the amount of water in the river changes a lot seasonally. To level the drought problems in the lower Pa Sak valley, in 1994 the construction of the Pasak Jolasit Dam (เขื่อนป่าสักชลสิทธิ์) in the Lopburi Province was started. The 4,860 meters wide and 31.5 m high dam keeps a total of 0.785 km³ of water. Additionally to the water management the dam also supplies about 6.7 MW of hydro-electric power.

Tributaries

Tributaries of the Pa Sak include the Lopburi, Muak Lek, Phung, Pa Daeng, Kong, Sonthi, Wang Chomphu, Na, Chun, Duk, Khon Kaen, Yai, Saduang Yai, Ban Bong, Tarang and Phaya Klang Rivers.

Pa Sak Basin

The Pa Sak drains a total land area of 16,291 square kilometers.

This Pa Sak Basin is part of the Chao Phraya Watershed.