Rio Macacu is a coastal river in the state of Rio de Janeiro, southeastern Brazil, draining the Serra do Mar mountain range into the Baía de Guanabara. It flows through the municipality of Cachoeiras de Macacu and forms part of the broader Guanabara-Guandu bioregion — one of the most ichthyologically diverse freshwater systems in the state. The river basin is notable for its remarkable fish diversity, harboring both widespread Atlantic Forest species and a number of endemic species found nowhere else on Earth. The headwaters originate in the steep, forested slopes of the Serra do Mar, giving the upper river a classic mountain stream character with clear, fast-moving water, while the lower reaches transition into slower, more lowland-influenced habitat as the river approaches the coastal plain. The RJ-116 highway runs near portions of the river, providing access to key stretches around Cachoeiras de Macacu.