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. It has a drainage basin of 10,448 km² (4,034 sq mi), the largest in the country, which covers almost one-fifth of the total area of the island. The real beginning of Mahaweli Ganga starts at Polwathura (at Mahawila area), a remote village of Nuwara-Eliya District in bank Nawalapitiya of Kandy District by further joining of Hatton Oya and Kotmale Oya. 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 natural deep-sea harbours in the world.

As part of Mahaweli Development programme 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.

There is a misconception in Sri Lanka that the Mahaweli starts in the Sri Pada mountain. The Mahaweli gets its source waters from Horton Plains in Kirigalpoththa and the Thotupola mountain range.