Diplomystes incognitus can be diagnosed from other species of the genus Diplomystes by having the following characters: skin of head, body, and fins densely covered by round, short papillae giving the skin a blackberry-like or verrucose aspect in large individuals; with a short head, slightly squarish and as long as broad (vs. slightly longer more triangular-shaped head); height of dorsal fin, ca. 20% of SL (range 17-25%) and triangularly-shaped (vs. slightly rhomboidal); maxilla with 7-9 teeth (vs. 8-13 in D. chilensis, 11-3 in D. nahuelbutaensis, and 12-19 in D. camposensis); with 10 infraorbital bones, as in D. nahuelbutaensis, but the dorsalmost compound bone is absent; urogenital pore and anus placed between posterior tips of pelvic fins as in D. chilensis (vs. urogenital pore and anus placed between pelvic fins or in between the distal tips of pelvics and anal fin); lack of pores of axillary gland with occasionally four on one side of body (vs. two or three pores) (Ref. 116749).