[3] This taxon is largely sympatric with the white-spotted spinefoot (Siganus canaliculatus) and these taxa are also very similar in appearance.

[7] Siganus fuscescens has a moderately slender, laterally compressed body, the standard length being 2.3 to 2.9 times its depth.

The dorsal profile of the head is weakly to notably concave over the eyes with either a blunt or a pointed snout.

The caudal fin is nearly emarginate in smaller individuals of less than 10 cm (3.9 in) standard length becoming forked in larger fish.

[2] The overall colour of this rabbitfish is greenish-grey to brown, fading to silvery on the lower body, and it has a large number of small light-bluish spots on the flanks, and a slender brown bar along the upper margin of the operculum.

[10] It is found as deep as 50 m (160 ft) in shallow coastal waters in algae, sea grass and coral or rocky reefs.

[2] When they arrive on coral reef flats, the larvae aggregate in schools with a normal size of 200 individuals, but may hold as many as 5,000.

[2] Siganus fuscescens, like other rabbitfishes, has venomous spines in the dorsal and pelvic fins.