Pseudancistrus kayabi is distinguished from all congeners except P. reus by having dark bars in caudal and dorsal fins (vs. with white spots in caudal and dorsal fins). It can be further diagnosed from all Pseudancistrus except P. nigrescens by having a dark brown body with whitish spots that fade along the posterior portion of the dorsal fin and forming mottled pattern (vs. either dark brown with conspicuous rounded spots not covering more than one plate in P. barbatus, P. corantijniensis, P. depressus, P. asurini, and P. zawadzkii or with dark brown bars in P. reus). It further differs from P. barbatus and P. depressus by having the snout with yellowish hypertrophied odontodes (vs. reddish-brown odontodes). Other characters useful for the identification of P. kayabi include the following: shorter pectoral spine, 22-30% SL (vs. 29-34% in P. nigrescens, 31-33% in P. zawadzkii, and 30-34% in P. barbatus); shorter dorsal-fin base, 20-28% SL (vs. 28-29% in P. nigrescens, 29-31% in P. zawadzkii, and 28-31% in P. barbatus); greater internares width, 13-19% HL (vs. 10.5-12.9% in P. nigrescens); head depth, 60-66% HL, greater than in P. nigrescens (56-57%) and in P. barbatus (41-53%) but smaller than in P. zawadzkii (67-73%); and a greater adipose-anal distance (17-25% SL vs. 15-17% in P. nigrescens and 12-15% in P. barbatus) (Ref. 99871).