A new species of the genus Adontosternarchus (Gymnotiformes, Apteronotidae) is described from samples collected during high water periods in a relatively restricted area within the Rio Purus basin, Amazonas, Brazil. Adontosternarchus duartei differs from congeners in a series of details of pigmentation, meristics, and morphometrics and represents the sixth species in the genus. A key to the species of Adontosternarchus is provided.