Sternopygus branco n. sp. is described from the Amazon River between its confluences with the Rios Japurá and Negro, and from the lower 100 km of the Rio Negro. This species is described using features of external morphology, meristics, pigmentation, osteology, and electric organ discharges (EODs). The new species is diagnosed by a very low EOD repetition rate (24–35 Hz vs approximately 40–300 Hz in congeners); very pale pigmentation on the entire body surface in live specimens; the absence of a pale lateral stripe along dorsal margin of anal-fin pterygiophores (present in adults and/or juveniles in all congeners); a slender body (body depth 8.3–10.9% length to end of anal fin vs 10.3–16.1% in congeners); posterior margins of cleithrum and supracleithrum short and robust. Sternopygus branco is found in the main river channels of whitewater and blackwater rivers and in adjacent side channels. It is not known from seasonally flooded forests or terra firme stream systems. Sternopygus branco is able to rapidly modulate the amplitude of its EOD.