Characidium mirim is distinguished from all congeners, except Characidium bahiensis, C. interruptum, C. nupelia, C. rachovii, C. stigmosum, and C. xavante by having an incomplete lateral line with 5–7 perforated scales (vs. lateral line with more than 15 perforated scales). The new species is further distinguished from all congeners, except C. nupelia, C. stigmosum, C. vestigipinne, and C. xavante by lacking an adipose fin. Characidium mirim is distinguished from Characidium nupelia and C. xavante by having a conspicuous dark midlateral stripe (vs. midlateral stripe absent or represented by a thin and inconspicuous dark line), nine to 11 dark transverse bars on the dorsum not extending ventrally onto side of body (vs. 12 to 18 dark transverse bars on dorsum and side of body) and lacking a dark blotch on the caudal peduncle (vs. dark blotch present). Characidium mirim is further distinguished from C. xavante by having 12 series of scales around the caudal peduncle (vs. 10). Characidium mirim is distinguished from C. stigmosum by having a conspicuous dark midlateral stripe (vs. stripe absent), the side of body without dark blotches (vs. dark blotches present), the fins hyaline (vs. generally dark), and 12 circumpeduncular scales (vs. 14). The new species differs from C. vestigipinne by the absence of blotches on pelvic and anal fins (vs. blotches present). (Ref. 93569).