Acanthochitona fascicularis has a relatively flat, extended oval shape with a shell length of up to 6 cm. Its width is half that of its length. The colour may be variable; being marbled with off white, grey, yellowish or brown. This species has a broad and spiny girdle with 18 dense tufts of long bristles (up to 1.5 mm long), 4 of which are in an arc around the head valve. The surface is covered with recumbent spines, making it feel velvety to the touch. The girdle is fringed with spines, up to 1 mm in length. Valves on the shell are strongly arched and, when viewed under a hand-lens, appear coarsely granular due to densely packed and evenly arranged dorsal papillae.