Rio Piraí is a significant river flowing through the state of Rio de Janeiro in southeastern Brazil, draining through a landscape shaped by the remnants of the Atlantic Forest — one of the most biologically diverse ecoregions on Earth. The river is part of the Middle Rio Paraíba do Sul bioregion, a richly complex watershed that supports an extraordinary variety of freshwater fish species. The Piraí watershed has been the subject of biodiversity surveys by the National Museum of Rio de Janeiro, which documented several species found only in this region, including at least one endangered fish species. The river feeds into a series of reservoirs, most notably the Represa Santana, which serves as a key fishing destination along the system. The surrounding landscape, though heavily impacted by centuries of agriculture, cattle ranching, and urban development — resulting in the loss of nearly 90% of the original Atlantic Forest cover — still harbors remarkable aquatic biodiversity, making Rio Piraí a destination of both recreational and ecological significance.