Adhemarius gannascus is a moth of the family Sphingidae. It is known from Mexico, Belize, Guatemala, El Salvador, Honduras, Nicaragua, Costa Rica, Panama, Colombia, Ecuador, Peru, Venezuela, Guyana, Suriname, French Guiana, Bolivia, Brazil, northern Argentina, southern Paraguay and Uruguay. The wingspan is 92–112 mm for males and 98–124 mm for females. Adults are on wing year round. The larvae have been recorded feeding on Ocotea veraguensis, Persea povedae, Persea americana and Nectandra salicina.