Lam Sonthi (, ) is a tributary of the Pa Sak River. It originates at the Ruak and Phanghoei mountains in the border area of Lam Sonthi district, Lopburi province and Si Thep, Phetchabun province.
The Lam Sonthi river, flowing through the border region of Lam Sonthi district in Lopburi province and Si Thep in Phetchabun province, originates from the Ruak and Phanghoei mountains.