The water scavenger beetle genus Chasmogenus Sharp, 1882 is reviewed in northeastern South America using an integrative approach that combines adult morphology and molecular data from the gene cytochrome c oxidase I (COI). Eighteen new species are described: Chasmogenus acuminatus sp. nov. (Brazil, French Guiana, Guyana, Suriname), C. amplius sp. nov. (Venezuela), C. berbicensis sp. nov. (Guyana), C. brownsbergensis sp. nov. (Suriname), C. castaneus sp. nov. (Venezuela), C. clavijoi sp. nov. (Venezuela), C. cuspifer sp. nov. (Venezuela), C. flavomarginatus sp. nov. (Venezuela), C. gato sp. nov. (Venezuela), C. guianensis sp. nov. (Suriname, Guyana), C. ignotus sp. nov. (Brazil), C. ligulatus sp. nov. (Suriname), C. lineatus sp. nov. (Venezuela), C. pandus sp. nov. (Brazil, French Guiana, Suriname), C. schmits sp. nov. (Suriname), C. sinnamarensis sp. nov. (French Guiana), C. tafelbergensis sp. nov. (Suriname), and C. undulatus sp. nov. (Guyana). We found genetic support for an additional new species in Guyana which is currently only known from females that we refer to as Chasmogenus sp. C. We examined the holotypes of the four species previously known from the region, and found that C. occidentalis García syn. nov. and C. yukparum García syn. nov. are conspecific with C. bariorum García, 2000 and are synonymized with that species, which is here redescribed. We redescribe C. australis García and expand the range of this species to include northern Brazil, Guyana, and French Guiana. All species are aquatic, with most being associated with forested streams and forest pools. Of the 21 species, more than half (11) are only known from a single locality indicating the genus may have many more micro-endemic species yet to be discovered in the region. Characters of the male genitalia are essential for confirming the identity of some species, consequently it is not always possible to make positive identifications of unassociated female specimens based on morphology alone. Habitus images are provided as well as a revised key to the genus for northeastern South America.