The Lempa River (Spanish: Río Lempa) is a 422-kilometre-long (262 mi) river in Central America.^[2] It is a transboundary river shared by El Salvador, Guatemala and Honduras.^[3]

Its sources are located in between the Sierra Madre and the Sierra del Merendón mountain ranges in southern Guatemala, near the town of Olopa.^[2] In Guatemala, the river is called Olopa River and flows southwards for 30.4 kilometres (18.9 mi) before entering Honduras and changing its name to Lempa River at 14°32′52″N 89°15′50″W / 14.547700°N 89.264002°W / 14.547700; -89.264002. In Honduras, it flows through the Ocotepeque Department for 31.4 kilometres (19.5 mi), and crosses the border with El Salvador at the town of Citalá (14°22′19″N 89°12′45″W / 14.371857°N 89.212439°W / 14.371857; -89.212439) in the Chalatenango Department. The river continues its course for another 360 kilometres (220 mi) in El Salvador, flowing in a generally southwards direction until it reaches the Pacific Ocean in the San Vicente Department.^[2][4] The river forms a small part of the international boundary between El Salvador and Honduras.

The river's watershed covers 18,246 km² (7,045 sq ml) of which 10,255 km² (3,959 sq ml), that is, 56.56% of the watershed territory, lie in El Salvador; 5,696 km² (2,199 sq ml) in Honduras; and 2,295 km² (886 sq ml) in Guatemala.^[1] 49% of El Salvador's territory is covered by the Lempa River basin,^[1] and 77.5% of the Salvadoran population lives in cities, towns, and villages located within the basin territory, including the capital city of San Salvador.^[4]

There are several hydroelectric dams along the river. In El Salvador, there is the Guayojo Dam, the Cerrón Grande Hydroelectric Dam, the 5 de Noviembre Dam, and the 15 de Septiembre Dam. The latter can be easily seen from the Pan-American highway.

• List of rivers of Guatemala
• List of rivers of Honduras
• List of rivers of El Salvador
• List of rivers of the Americas by coastline