A new species of ghost knifefish is described from the Rio Uatumã at Cachoeira do Miriti, Amazon basin, Brazil. It is distinguished from all species of Apteronotus by the absence of a clear stripe from the chin to the dorsal portions of the head and/or dorsum, a reduced number of anal-fin rays (118–122), and by having four branchiostegal rays. More specifically, the new species resembles Apteronotus quilombola by the low number of anal-fin rays; however, the number of branchiostegal rays and several additional counts and measurements differentiate the species. Furthermore, the new species co-occurs with A. lindalvae, and it can be distinguished by the number of premaxillary teeth. The occurrence area of the new species was highly impacted after the construction of Balbina dam. Thus, considering the threats to the species and the restricted area of distribution, its conservation status was assessed as critically endangered.