A new species of Bathyaethiops is described from the Lékoumou River in the Kouilou–Nairi basin of the Republic of Congo. Bathyaethiops atercrinis, new species, is distinguished from its three central African congeners in the possession of 21–24 scales in longitudinal series vs. 28–42 scales, and from Brachypetersius altus by the possession of a short, abridged lateral line consisting of only six or seven pored scales vs. a complete lateral line of 20–28 pored scales. The species is unique among congeners and related taxa in displaying marked sexual dichromatism and dimorphism of the supporting skeleton of the anal fin. The presence of B. atercrinis in the Lékoumou represents the first record of the genus in the Lower Guinean ichthyofaunal province and the first occurrence outside of the Congo basin.