Lake Piso, also known as Lake Pisu and Fisherman's Lake, is an oblong tidal lagoon in Grand Cape Mount County in western Liberia, near the town of Robertsport. At an area of 103 km2 (40 sq mi), it is the largest lake in Liberia. Its name originates from a local term meaning "pigeon's hole" a reference to the flocks of pigeons that once came to Lake Piso for water. The brackish water lake borders the Cape Mount peninsula, and is connected to the Atlantic Ocean by a narrow inlet known locally as the "bar mouth". Various rivers empty into Lake Piso, including Mole Creek and Mafa River, which discharges at the 'bar mouth'. There are a number of islands in the lake, including Massatin Island, which offered a haven for some Liberians during the Liberian Civil War. During World War II, Lake Piso functioned as a military base for Allied seaplanes.
Lake Piso overflowed its banks at least twice in 1998, causing heavy flooding in Garwular District. Flooding in August 1998 occurred due to a blockage of the narrow inlet connecting the lake to the Atlantic Ocean, damaging some homes. Additional flooding in September and October, brought about by heavy rains, affected over twenty settlements, causing at least ten deaths, leaving thousands homeless, and forcing others to leave their homes. The floods led the government of Liberia to declare the area a "disaster zone".