A new species of the genus Moenkhausia is described from the middle portions of the Rio Tocantins and tentatively from the Río Apure in Venezuela, Río Orinoco drainage. It is diagnosed by the presence of a dark spot in the half proximal length of the upper caudal-fin lobe, and by the presence of a large, irregularly shaped humeral spot located above the sixth to ninth perforated lateral line scale which is more posteriorly positioned along the body side in relation to other members of the genus. The distribution pattern of the new species is discussed. Copyright © 2007. Magnolia Press.