It has a deep, highly compressed, oval-shaped body covered in rough, sandpaper-like skin. A single strong, barbed spine projects from the first dorsal fin above the eyes, while the pelvic fins are absent and replaced by a small skin flap. Its colour varies from brown to olive-grey, with darker reticulated markings or spots across the body. The dorsal and anal fins are often tinged with yellow or orange.

It looks like Aluterus monoceros (Unicorn leatherjacket), which is often confused with Stephanolepis diaspros, particularly in the Mediterranean, where both have been introduced.