The Ölfusá is a river in Iceland. It is formed when the rivers Hvítá and Sog combine, just north of the town of Selfoss, and flows for 25 km into the Atlantic ocean. It is Iceland's largest river with an average flow of 423 m³/s. Its discharge area is 5760 km². The Ölfusá has a strong salmon fishing industry. The Flói Nature Reserve is located on its eastern shore near where it meets the sea.