Fucus is a genus of brown alga (seaweed) that lives in the intertidal zones of rocky shores. A common species found on the Atlantic coasts of Europe and North America is Fucus vesiculosus or bladder wrack. On the Pacific coast of North America, the most common species is Fucus gardneri.

The thallus is perennial with an irregular or disc-shaped holdfast. The erect portion of the thallus is dichotomous or subpinnately branched, flattened and with a distinct midrib. Gas-filled pneumatocysts (air-vesicles) are present in pairs in some species, one on either side of the midrib. The erect portion of the thallus bears cryptostomata and caecostomata (sterile surface cavities). The base of the thallus is stipe-like due to abrasion of the tissue lateral to the midrib and it is attached to the rock by a holdfast. The gametangia, develop in conceptacles embedded in receptacles in the apices of the final branches. They may be monoecious or dioecious.