Moenkhausia plumbea is distinguished from all congeners, except M. chlorophthalma and M. petymbuaba, by the presence of large dark blotches on the anterior to the central portions of each scale along the seven dorsalmost longitudinal series (vs. pigmentation absent or, when present, concentrated posteriorly along the border of scales and forming a reticulate pattern in the body). Moenkhausia plumbea differs from both M. petymbuaba and M. chlorophthalma in possessing a dark longitudinal stripe along the eye; and is further separated from M. petymbuaba in lacking a very conspicuous dark midlateral stripe; and from M. chlorophthalma in possessing eight or nine longitudinal rows of large dark blotches (vs. seven), 33-36 lateral line scales (vs. 25-28), five scales between lateral line and dorsal fin (vs. four), 4-9 anal-fin base scales (vs. 7-12), and usually 6 branched pelvic-fin rays (vs. 7) (Ref. 84462).