Pteromalus cassotis is a species of parasitic wasp in the family Pteromalidae that parasitizes the chrysalides of monarch butterflies.

They are gregarious parasitoids, meaning a single female lays many eggs in a single host.

Research into this species has documented that up to 425 adult wasps can emerge from a single chrysalis.

[2][3][4] Maximum entropy models suggest that the natural habitat of this species encompasses the continental United States, southern Canada and parts of Mexico; areas inhabited by the caterpillars of monarch butterflies, which are the larvae's hosts.

[5] This chalcid wasp-related article is a stub.