Isognathus australis is a moth of the family Sphingidae.

The forewing upperside is unicolorous brown, shaded with white, except for a yellow band along the posterior margin and the faint traces of a distal marginal band.

The basal area of the hindwing underside is yellow, the rest is brown with a bluish tinge, becoming black towards the anal angle.

The larvae have been recorded feeding on Plumeria species.

This Dilophonotini-related article is a stub.