Rio Roncador, also known as Rio Santo Aleixo, is a river located in the state of Rio de Janeiro, Brazil, draining the oceanic slopes of the Serra do Mar and flowing into the Guanabara-Sepetiba bioregion — one of the most biologically rich freshwater systems in southeastern Brazil. This river is part of a network of streams and rivers that originate in the Atlantic Forest and empty toward Guanabara Bay and Baía de Sepetiba, flowing through a landscape shaped by dramatic geological formations characteristic of the Serra do Mar mountain range. The surrounding Atlantic Forest environment gives the river a lush, densely vegetated riparian corridor, and the watershed is recognized as a hotspot for endemic freshwater fish species found nowhere else on Earth.