The Nakhon Nayok River (Thai: แม่น้ำนครนายก, rtgs: Maenam Nakhon Nayok, Thai pronunciation: [mɛ̂ːnáːm náʔkʰɔːn naːjók]) originates in the Khao Yai National park. It flows heading to Southwest, passes the districts Mueang Nakhon Nayok, Ban Na and Ongkharak of Nakhon Nayok Province. It then joins the Prachin Buri River to become the Bang Pakong River in Ban Sang district, Prachinburi Province at Pak Nam Yothaka.