Balanus trigonus is a sharply conical barnacle with steep sides. A shell made up of six overlapping plates. These plates are often folded forming broad vertical grooves. they are usually purplish-pink or mottled with that coloration. They encircle a central, roughly triangular opening (orifice). Shell surface shows horizontal striations. Inside, the barnacle has two prominent valves—the scutum and the tergum—that protect its appendages.