The Maha Oya (Sinhala: මහ ඔය, lit. 'Great River') is a major watercourse in the Sabaragamuwa Province of Sri Lanka. It measures approximately 134 km (83 mi) in length. It runs across four provinces and five districts. The Maha Oya has 14 water supply networks to serve the need of water and more than 1 million people live by the river.

Its catchment area receives approximately 3,644 million cubic metres of rain per year, and approximately 34 percent of the water reaches the sea. It has a catchment area of 1,510 square kilometres.