The Rio Marapanim is a river in the state of Pará, Brazil, rising in the municipality of São Francisco do Pará and flowing generally northward before entering the Atlantic Ocean just past the town of Marapanim, Pará, which sits on its left bank. The river's mouth is characterized by mud flats, and the broader estuary opens into Marapanim Bay near Maiandeua Island.

The surrounding area has a low population density of approximately 18 people per square kilometre. The climate is monsoon in character, with an average annual temperature of 24°C (75°F). September is the warmest month at around 25°C (77°F), while January is the coolest at roughly 22°C (72°F). Annual rainfall averages 3,146 millimetres, with March being the wettest month at 699 millimetres and October the driest at just 12 millimetres. Paddlers planning a trip should factor in this pronounced wet-dry seasonality, as March conditions will differ substantially from those in October.

The eastern side of Marapanim Bay is protected by the Maracanã Marine Extractive Reserve (30,179 hectares, established 2002) and, further south, the Cuinarana Marine Extractive Reserve. The western side falls within the Mestre Lucindo Marine Extractive Reserve (26,465 hectares, established 2014). Paddlers should be aware that these are designated marine extractive reserves, and any applicable regulations governing access or activity within their boundaries would need to be confirmed with the relevant Brazilian authorities before planning a trip.