Lake Arenal is an artificial lake in Costa Rica, located in the northern highlands of the country. It was enlarged to its present size as a part of a 1979 hydroelectric project by the Costa Rican government to provide electricity to the country.

It is located in the Arenal Tilaran Conservation Area, and is close to the Arenal Volcano and Monteverde cloud forest. The lake is approximately 30 km long and almost 5 km at its widest point, making it the largest lake in Costa Rica at 85-square-kilometre (33 sq mi). Its depth varies between 30 and 60 meters (100–200 feet) depending on the season. The town of Arenal was relocated to higher ground when the lake was expanded in 1979. The old towns of Arenal and Tonadora now lie abandoned at the bottom of the lake, with the new town of Arenal existing to the northeast on the lake.