Demini River is a river in the Amazon rainforest of the state of Amazonas, Brazil. It is a tributary of the Rio Negro.

The Demini River rises in the Serra do Aracá State Park, a 1,818,700 hectares (4,494,000 acres) conservation unit created in 1990. It flows southwest through the park, then flows south to join the Rio Negro. The Toototobi River is a tributary of the upper part of the Demini near the headwaters of the Orinoco near the border of Venezuela. The Yanomami índios are native to this area.