The Rio Mearim is a major river in Maranhão state, northern Brazil, running approximately 771 to 800 kilometers from its origins in the southern part of the state northward to the Baía de São Marcos, where it forms a common estuary with the Rio Pindaré before reaching the Atlantic Ocean. The river's primary water source is rainfall, and it flows through the marshlands of the Atlantic Coastal Plain for much of its length.

The upper and middle courses of the Mearim are characterized by rapids, making those sections technically demanding and generally unsuitable for straightforward navigation. Only the lower sections of the river are considered navigable. Paddlers considering the lower Mearim should be aware of the pororoca, a tidal bore that occurs in the river's lower reaches. Tidal bores can produce powerful, fast-moving walls of water that travel upstream and pose a serious hazard to anyone on the water who is unprepared for them. The pororoca on the lower Mearim has attracted surfers, which reflects the bore's significant size and force.

The river defines the southern boundary of the Tocantins–Araguaia–Maranhão moist forests ecoregion, indicating that the surrounding landscape transitions between distinct ecological zones. Prospective paddlers should seek local knowledge regarding access points, recommended routes, facilities, and seasonal water conditions before planning a trip.