Tometes siderocarajensis can be diagnosed from all its congeners by dense pigmentation on the distalmost portion of the pelvic-fin rays or the entirety of the fin (vs. pelvic fin hyaline or pale, or with few scattered dark chromatophores along distal portion of rays). In addition, it differs from all congeners, except from T. camunani and T. kranponhah, by having more circumpeduncular scale rows 39?41 (vs. 38 or less), and from T. kranponhah and T. trilobatus by having 1st and 2nd labial premaxillary teeth laterally spaced (vs. 1st and 2nd labial premaxillary teeth with lateral contact). It is further distinguished from T. trilobatus by having more perforated lateral line scales 74±84 (vs. 58±72); from T. makue by having more spines on ventral keel 11±17 prepelvic spines and 26±33 total spines (vs. 0±9 and 10±23, respectively); and from T. lebaili by having a terminal to gently upturned mouth and invariably 5 dentary teeth (vs. a markedly upturned mouth and 6±7 dentary teeth) (Ref. 116762).