The Rio Açungui is a river in Paraná state, southern Brazil, rising in the foothills of the Serra de São Luiz do Purunã near the district of São Luiz do Purunã in Campo Largo, roughly to the northwest of Curitiba. From its source, the river flows in a generally northeast direction, passing through Campo Largo, Itaperuçu, and Rio Branco do Sul before joining the Rio Ribeira, which continues east into São Paulo state. The confluence of the Açungui and the Ribeirinha rivers marks the headwaters of the Ribeira de Iguape River, all within approximately 100 kilometers of Curitiba.

The Açungui valley is characterized by steep terrain with numerous small canyons. The river contains rapids, most notably in the Canyon do Acalantilado, an excellent location for rafting. The steep valley walls and proximity to the Açungui National Forest have contributed to a well-preserved natural environment along much of the river's course.

The river basin covers approximately 1,800 square kilometers and is considered the last natural reserve of unpolluted water available to supply the metropolitan region of Curitiba. The Açungui National Forest, established in 1944 and covering 3,728 hectares, borders the river and protects stands of araucaria trees. This combination of protected forest and relatively clean water gives the river a largely undeveloped character through much of its course.

Given the river's role as a critical drinking water source for the Curitiba metropolitan area, paddlers should be aware that water quality protection is a significant concern in this basin, and any applicable regulations related to that designation should be verified before planning a trip.