The Pa Sak River 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 through the eastern part of Lopburi Province and Saraburi Province, until it joins the Lopburi River northeast of Ayutthaya Island, before it runs into the Chao Phraya River southeast of Ayutthaya near Phet Fortress. It has a length of 513 km (319 mi) and drains a watershed of 16,291 km2 (6,290 sq mi). The annual discharge is 2.4 km3 (0.58 cu mi).