This article describes two new species of Monomorium from Brazil. One, M. delabiei n. sp. is reminiscent of the genus Megalomyrmex, although it lacks the transverse propodeal carina characteristic of that genus. The other, M. inusuale n.sp., is even more interesting: given the current concepts in Solenopsidini, it could be treated as a new genus within the tribe due to its distinctive morphological characteristics (mandibular configuration, vestibulate propodeal spiracle, propodeal carinae, micropegs on the last tergum). Broadening the limits of Monomorium requires including Nothidris, Phacota and Epelysidris as junior synonyms (syn. nov.). A working key to Neotropical species of Monomorium is provided along with taxonomic notes.