Steatogenys ocellatus sp. nov. is described from the lowland Amazon basin at localities near Tefé (Brazil) and Iquitos (Peru). This species is described using features of external morphology, meristics, pigmentation, osteology, and the electric organ discharge. The new species is diagnosed by (1) a relatively large maximum body size (maximum length to end of anal fin 320 mm vs 172 mm in other species); (2) a distinctive round or oval spot on the pectoral-fin base; and (3) irregular dark blotches on the pectoral fin forming a mottled pattern. Several morphometric and meristic features also unambiguously distinguish S. ocellatus sp. nov. from the two known congeners, Steatogenys elegans and Steatogenys duidae. Steatogenys elegans is common in riverine and lacustrine habitats throughout the lowland Amazon and Orinoco basins and the Guyanas. Steatogenys duidae is found in terra firme forest creeks throughout the Amazon and Orinoco basins. Steatogenys ocellatus sp. nov. inhabits static or slow-flowing habitats such as seasonally flooded forests in low conductivity blackwater systems. In the Tefé region it is found in sympatry with S. elegans and S. duidae but in syntopy only with the former.