Mottled sculpin are small, stout fish with relatively flattened bodies. They have round snouts and are cryptically colored, with brown to black mottling on their backs, sides, and fins and whitish bellies. They have 2 dorsal fins, the first with 6 to 9 soft spines and the second with 17 to 19 rays. There is a small black spot on the first part of the first dorsal fin, which helps to distinguish them from round gobies, and a larger spot on the back of the first dorsal fin. They do not have obvious scales, although they have a line of small prickle-like scales below their lateral line, which is incomplete and ends just under the second dorsal fin. Males are slightly larger than females and during the breeding season males have a dark band on the first dorsal fin and a broad, orange band on the edge of the fin.
Range length: 76 to 102 mm.
Other Physical Features: ectothermic ; heterothermic ; bilateral symmetry
Sexual Dimorphism: male larger; male more colorful