Leptotes cassius, the Cassius blue or tropical striped blue, is a butterfly of the family Lycaenidae.

The butterfly species has an important role in Marisha Pessl's 2006 novel Special Topics in Calamity Physics.

Food plants on record are Amorpha crenulata, woolly rattlepod (Crotalaria incana), Galactia regularis and lima bean (Phaseolus lunatus).

It can also successfully develop feeding on Cape leadwort (Plumbago auriculata) or doctorbush (P. scandens), which (among the eudicots) are not closely related to its usual food plants.

[3] L. c. theonus (and consequently L. cassius itself) is the type species of the genus Leptotes.