Onega comprises nine valid species distributed in South American countries, Bolivia, Colombia, Ecuador, Paraguay and Peru, commonly at high altitudes. The genus has as diagnostic characteristics the transition crown-frons with transversal carina; crown and superior portion of frons with concavities; pronotum wider than transocular width of head; and paraphysis, when present, as a median esclerite. The present paper describes Onega musa sp. nov., from Ecuador and Peru, which can be distinguished from other Onega species by: body mostly yellow, with brown maculae distributed on dorsum; posterior margin of male pygofer serrate, with long microsetae on the basiventral margin; aedeagus with shaft bisinuate with dorsal acute preapical process; female sternite VII with posterior margin slightly convex; and first valvula of ovipositor with 38 noncontiguous teeth. Intraspecific morphological variations are discussed.