The Lempa River (Spanish: Río Lempa) is a 422-kilometre-long (262 mi) river in Central America.

Its sources are located in between the Sierra Madre and the Sierra del Merendón mountain ranges in southern Guatemala, near the town of Olopa. In Guatemala, the river is called Río Olopa 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 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 department of Chalatenango. 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 department of San Vicente. The river forms a small part of the international boundary between El Salvador and Honduras.

The river's watershed covers 18,246 km (7,045 sq ml) of which 10,255 km (3,959 sq ml), of which 56.56% lie in El Salvador, 5,696 km (2,199 sq ml) in Honduras and 2,295 km (886 sq ml) in Guatemala. Forty-nine percent of El Salvador's territory is covered by the Lempa river basin, and 77.5 percent of the Salvadoran population lives in cities, towns, and villages that are in its basin, including the capital city of San Salvador.

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, which 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