The Rio Mocajuba is a short river in the state of Pará, Brazil, flowing northward before emptying into the Atlantic Ocean. The village of São João da Ponta sits on the left bank. The river passes through an area flanked by several protected areas: the Mocapajuba Marine Extractive Reserve lies to the west, the São João da Ponta Extractive Reserve to the southeast, and the Mãe Grande de Curuçá Extractive Reserve to the east. An arm of the river, the Furo Maripanema, branches off to the northeast and connects with the Curuçá River. The Barreto River enters as a left-bank tributary.

The surrounding landscape is almost entirely forested, with a low population density of approximately 19 people per square kilometer. The climate is monsoonal, with an annual average temperature of 23°C and average annual rainfall of 3,146 mm. March is the wettest month, receiving around 699 mm of rain, while October is the driest at roughly 12 mm. Temperatures range from about 22°C in February to 24°C in July. Paddlers should factor the pronounced wet and dry seasons into trip planning, as water levels and conditions will vary considerably between March and October.