The Sarstoon River (Spanish: Río Sarstún) is a river in the Toledo District of Belize. It forms the country's southern boundary with Guatemala.
The river's sources are in neighbouring Guatemala. It flows through the Guatemalan departments of Alta Verapaz and Izabal into Belize and through Toledo to the Caribbean Sea.
The Sarstoon is the internationally accepted southern border between Belize and Guatemala, although as part of the Belizean–Guatemalan territorial dispute since 1999 Guatemala has claimed Belizean territory south of the Sibun River.