Hypopygus is a genus of South American gymnotiform knifefishes native to the Amazon, Orinoco and upper Paraguay basins, as well as rivers in the Guianas.

[1][2] They are often common, and found near submerged roots, aquatic vegetation and leaf-litter in streams, edges of rivers and floodplains.

[1] They are regularly found among vegetation in floating meadows, a habitat that often contains little oxygen, but they are well-adapted to this.

[1][2][4] They generally resemble the related Steatogenys, but are smaller,[5] reaching up to 5.9–12.2 cm (2.3–4.8 in) in total length depending on the exact species of Hypopygus.

[1] Hypopygus has traditionally been included in the family Hypopomidae,[8] but a comprehensive molecular study from 2015 showed it belongs in Rhamphichthyidae,[9] and this has been followed by recent authorities.