Amphibalanus improvisus has a conical to slightly cylindrical shell made of white to greyish calcareous material, formed by six fused plates. The shell opening is slightly toothed and broader than half the shell’s height. The plates are mostly smooth. The radii are narrow and white. The operculum, which seals the opening that includes a prominent adductor ridge on the scutum. A tergum with a blunt tip and a slender spur. The shell’s base is flat, calcareous, and marked with radial canals.