Mycocepurus castrator is a species of parasitic ant, in the genus Mycocepurus, native to Brazil. Described in 2010, the species is a workerless and obligate parasite of the related ant Mycocepurus goeldii. It is known only from Rio Claro, Brazil, and has only been found in nests of M. goeldii.M. castrator and its host are closely related and diverged recently, around 37,000 years ago. They evolved in the same geographic region, making the parasite–host pair an example of sympatric speciation.