Isognathus allamandae is a moth of the family Sphingidae. It is found from Brazil to Venezuela. The wingspan is 68 mm for males and 78 mm for females. It is similar to Isognathus australis but has narrower wings and the forewing upperside is more uniform grey. There is a yellow band running along the inner margin of the forewing upperside. The basal area of the hindwing underside is yellow. There are probably multiple generations per year. Adults have been recorded in June.