Like other members of the serrasalmid family, Myloplus nigrolineatus is characterized by a deep, elliptical, and laterally compressed body, giving it a shape reminiscent of an elongated disc.

Breeding females show irregular orange and dark grey blotches scattered on flanks.

[1] Myloplus nigrolineatus is primarily distinguished from its cogeners by the black row of lateral line scales for which it is named.

Additional morphological characteristics that differentiate M. nigrolineatus from other members of its genus include the presence of 25 to 29 branched rays in the dorsal fin, and a greater number of lateral line scales (98-120 vs. fewer than 98 in other species).

[1] Myloplus nigrolineatus is widely distributed throughout the Amazon basin, where it inhabits slow-flowing backwaters and lakes.