Marcusenius wamuinii, a new large-scaled Marcusenius species, is described from the Mangroves National Park (MNP), a protected area situated at the mouth of the Lower Congo basin and its surroundings in the DR Congo. It can be distinguished from all its congeners based on the following unique combination of characteristics: 8 circumpeduncular scales, 27-31 anal-fin rays, 22-25 dorsal-fin rays, 19-22 scales between dorsal and anal fin, 46-53 lateral line scales, and a slender caudal peduncle (depth 4.4-5.9 % SL). Its status as a distinct species is additionally confirmed by genetic data from the mitochondrial cytochrome b gene. This is the first new fish species discovered in the MNP, highlighting the importance of freshwater conservation in this area in which the fish fauna is still poorly known