Balanus nubilus, commonly called the giant acorn barnacle, is the world's largest barnacle, reaching a diameter of and a height of up to , and containing the largest known muscle fibres.
Balanus nubilus is a northeast Pacific species that ranges from southern Alaska to Baja California. It is frequently found growing on rocks, pier pilings and hard-shelled animals at depths of up to . Like other acorn barnacles, B. nubilus is a filter feeder; it, in turn, is sometimes eaten by sea otters, sea stars, crabs and the Native Americans of the Pacific Northwest. Abandoned shells of B. nubilus are used by the crab Glebocarcinus oregonensis for shelter.