Soromonichthys may be diagnosed from all other Hypostominae by having a unique pattern of plate loss on the snout: Soromonichthys has naked skin covering its anteriormost snout margin, extending posteromedially over the mesethmoid to level approximately coequal with anterior bony rim of nostrils, and posterolaterally to about midlength of snout; plated region restricted to column along either side of mesethmoid that becomes broadly expanded posterolaterally to snout margin. Only three other Hypostominae genera have a loss of snout plates: Ancistrus, Chaetostoma, and Paulasquama. Ancistrus differs from Soromonichthys in having the naked region extending more posteriorly along the lateral margins of the snout and more evenly expanded posteriorly to near midlength of the snout (females) or beyond (males); all Ancistrus have plates along the posterolateralmost margins of the snout as well as medial plates over the posterior mesethmoid. Chaetostoma is similar to Ancistrus but with less developed or absent sexual dimorphism. Paulasquama differs in having naked region confined to bilaterally symmetrical patches on either side of the mesethmoid, each extending posteriorly to the anterior rim of the nostril; plates covering anterior and lateral margins of the snout and extending posteromedially over the mesethmoid continuous with posterior head plates. Soromonichthys is further distinguished from Ancistrus, Chaetostoma, and Paulasquama by having body coloration consisting of thin irregular light yellow to gold bars on a base color of irregularly mixed moderate to dark green (vs. coloration lacking thin light colored bars); from Ancistrus by lacking tentacles emergent from unplated regions of the snout, by having nasal no wider than canal (vs. nasal wide and almost square); and from Chaetostoma by having seven dorsal-fin rays (vs. eight to ten) (Ref. 87303).