Lithoxus raso is distinguished from all its congeners, with the exception of L. pallidimaculatus, by having large light spots all over the body, spots similar in size with orbit diameter, (vs. light spots over the body absent or light spots very small, similar to size of a tooth, covering only the dorsal portion of the head in L. boujardi); from L. surinamensis and L. pallidimaculatus by possessing an adipose fin (vs. adipose fin absent); from L. lithoides and L. jantjae by having five branched rays in the anal fin (vs. four); and from L. jantjae by having 14 branched rays in the caudal fin (vs. 12). In addition, Lithoxus raso is distinct from L. bovallii by having a lower premaxillary tooth cup length, 3.3-4.0% HL (vs. 4.2-10.4% HL); from L. boujardi by having a greater adipose-spine length, 7.2-8.1% SL (vs. 3.6-4.7% SL), and a greater dorsal adipose-caudal distance, 13.1-14.6% SL (vs. 10.3-11.7% SL); from L. planquettei by smaller caudal peduncle depth, 8.4- 8.6% SL (vs. 9.7-10.5% SL); and from L. stocki by having an enlarged and widened cusp of medial premaxillary teeth (vs. cusps of all premaxillary teeth similar in size, not enlarged), and by having a shorter cleithral width, 30.9-31.2% SL (vs. 32.6-34.2% SL) (Ref. 118148).