An intertidal barnacle with six solid wall plates, an oval-shaped operculum opening, and a membranous base. Juveniles have a kite-shaped opercular opening. The rostral plate is relatively narrow, plates are of roughly equal size, and its rostral plate is not fused with the rostrolateral plates. Usually conical in shape, however when crowded may become tubular. It may reach a diameter of approximately 14 mm, depending on habitat, food availability and level on the shore. The tissue inside the opercular aperture is bright blue with black and orange markings.Before 1976 Chthamalus montagui
was considered a variety of Chthamalus stellatus
, but in 1976 was identified as a distinct species, due to differences in its vertical zonation on the shore and morphology, particularly in the shape of the opercular plates, setation of the smaller cirri. (Southward, 1976).