Rio Guapiaçu is a river located in the state of Rio de Janeiro, Brazil, flowing through the Serra do Mar mountain range in southeastern Brazil. It forms one of the two main sub-basins of the Guapi-Macacu drainage system, alongside the Guapimirim sub-basin, and ultimately drains into Guanabara Bay through a shared mangrove area. The river originates in the highlands of the Serra do Mar, giving it the character of a clear-water mountain stream in its upper reaches, with fast-flowing, high-oxygen waters running over rocky and boulder-strewn substrate. As it descends toward the coastal lowlands, the river transitions in character, eventually reaching the mangrove-fringed estuary near Guanabara Bay. This diverse range of habitats — from swift upland streams to slower lowland reaches and estuarine zones — supports a variety of fish species and offers anglers a range of fishing experiences along its course.