The Mahaweli River (Sinhala: මහවැලි ගඟ, literally "Great Sandy River"; Tamil: மகாவலி ஆறு [mahawali gangai]), is a 335 km (208 mi) long river, ranking as the longest river in Sri Lanka. Its drainage basin is the largest in the country, and covers almost one-fifth of the total area of the island. The river reaches the Bay of Bengal on the southwestern side of Trincomalee Bay. The bay includes the first of a number submarine canyons, making Trincomalee one of the finest deep-sea harbors in the world. The river and its tributaries are dammed at several locations to allow irrigation in the dry zone, with almost 1,000 km² (386 sq mi) of land irrigated. Production of hydroelectricity from six dams of the Mahaweli system supplies more than 40% of Sri Lanka's electricity needs. One of the many sources of the river is the Kotmale Oya.

Coordinates: 8°28′N 81°14′E / 8.467°N 81.233°E / 8.467; 81.233