Itaipu Reservoir, Reservoir on the Brazil–Paraguay border, Paraná state.

The Itaipu Reservoir is a 1,350 km² artificial lake on the Paraná River, formed in 1982 by closing the diversion channel of the Itaipu Hydroelectric Plant. It lies on the Brazil–Paraguay border, with 770 km² on the Brazilian side and 580 km² on the Paraguayan side. Its 66 islands are divided into 44 Brazilian and 22 Paraguayan.

The reservoir was formed in 1982 following the construction of the Itaipu Dam, a project that began in the early 1970s as a bilateral agreement between Brazil and Paraguay for renewable energy development.

The name Itaipu derives from the Tupi language and translates to noise of the river of stones, referring to the turbulent waters of the Paraná River before the reservoir's creation.

The reservoir's formation submerged the Guaíra Falls and supports fifteen municipalities along its shores, including Foz do Iguaçu, Santa Helena, and Guaíra in Paraná state.

The reservoir drowned the Sete Quedas falls, a series of cascades on the Paraná where it dropped from the basalt plateau toward the Platine plain; the flooding in 1982 went ahead despite protests from environmentalists. The name Itaipu came from a small island that stood near the dam site.