Isognathus leachii is a moth of the family Sphingidae.

It is known from southern Panama and from Colombia, Venezuela, Brazil, Ecuador, Bolivia and Argentina.

There are indistinct bands on the upperside of the abdomen and an elongate patch on the forewing upperside.

The inner edge of the dark brown marginal band of the hindwing upperside is distinctly and almost regularly dentate and not constant in width.

They have long tails and are very colourful, suggesting they are unpalatable to birds.