Scorpaena cocosensis is a small species of scorpionfish and reaches a maximum published total length of . There are 12 spines and 8 soft rays in the dorsal fin while the anal fin has 3 spines and 5 soft rays. The chest is covered with well exposed scales. The pectoral fin has 22 fin rays. There are strong ridges in the intraorbital space which start immediately to the rear of the nasal spine and split at both ends when viewed from above. The occipital pit is shallow and is bordered by a bony ridge to its front. They have a large head and large eyes. The background colour is red mottled with white blotching. The operculum has dark red spots and the pectoral fin is red with a dusky tip. There a retro diagonal red bars on the soft rayed part of the dorsal fin and 2 dark red bars on the caudal fin.
