The dwarf Melocactus sergipensis, sp. nov., is described from a single lowland locality in the NE Brazilian state of Sergipe. It is compared with members of the M. VIOLACEUS species group, from which it may have been isolated on calcareous rocks within a former dense, high caatinga forest. Its longer term survival in the wild is in serious doubt, since the surrounding habitat has been converted for agriculture, and thus it is accorded the IUCN Red List status of Critically Endangered.