The Nakhon Nayok River (Thai: แม่น้ำนครนายก, RTGS: Maenam Nakhon Nayok, Thai pronunciation: [mɛ̂ːnáːm náʔkʰɔːn naːjók]) originates in Khao Yai National Park. It flows southwest, passes through Mueang Nakhon Nayok District, Ban Na District, and Ongkharak District of Nakhon Nayok Province. It then empties into the Bang Pakong River in Ban Sang District, Prachinburi Province at Pak Nam Yothaka. The river is 130 kilometres (81 mi) long.

13°54′N 101°10′E / 13.900°N 101.167°E / 13.900; 101.167