Ascophyllum nodosum is a large, common cold water seaweed or brown alga (Phaeophyceae) in the family Fucaceae. Its common names include knotted wrack, egg wrack, feamainn bhuí, rockweed, knotted kelp and Norwegian kelp. It grows only in the northern Atlantic Ocean, along the north-western coast of Europe (from the White Sea to Portugal) including east Greenland and the north-eastern coast of North America. Its range further south of these latitudes is limited by warmer ocean waters. It dominates the intertidal zone. Ascophyllum nodosum has been used numerous times in scientific research and has even been found to benefit humans through consumption.

Ascophyllum nodosum is the only species in the genus Ascophyllum. The original name (basionym) was Fucus nodosus Linnaeus 1753. The species was transferred to the genus Ascophyllum (as Ascophylla) by Stackhouse (Papenfuss 1950), under the name Ascophyllum laevigata (Guiry and Guiry 2020). The combination Ascophyllum nodosum was made by Le Jolis (1863).

Ascophyllum nodosum has long tough and leathery fronds, irregularly dichotomously branched fronds with large, egg-shaped air bladders set in series at regular intervals along the fronds and not stalked. The air bladders create a way for fronds broken by wave exposure or other causes to be dispersed and regrow in other areas. While the fronds can reach up to 2 m, the length depends on wave exposure: the length increases with water velocity until a certain point, then decreases as waves become more intense. The fronds can reach 2 m in length and are attached by a holdfast to rocks and boulders. In rare cases, the fronds can reach 6m long. The fronds are olive-green, olive-brown in color and somewhat compressed, but without a midrib. Young shoots are yellow, however darken with age.

Each individual plant is dioecious, either male or female. The gametes are produced in the spring in conceptacles embedded in yellowish receptacles on short branches. A year after the plant is fertilized and forms a zygote, the first frond grows, and at the beginning of year 2, an air bladder forms, which creates a way to age the plants.

Several different varieties and forms of this species have been described, including the two below.

Ascophyllum nodosum is found mostly on sheltered sites on shores in the midlittoral, where it can become the dominant species in the littoral zone.

The species is found in a range of coastal habitats from sheltered estuaries to moderately exposed coasts, and often it dominates the intertidal zone (although subtidal populations are known to exist in very clear waters). However, it is rarely found on exposed shores, and if it is found, the fronds are usually small and badly scratched. This seaweed grows quite slowly, 0.5% per day, carrying capacity is about 40 kg wet weight per square meter, and it may live for 10–15 years. It may typically overlap in distribution with Fucus vesiculosus and Fucus serratus. Its distribution is also limited by salinity, wave exposure, temperature, desiccation, and general stress. It may take approximately five years before becoming fertile. Ascophyllum nodosum is an autotroph, meaning that it makes its own food by photosynthesis, like other plants and algae. The air bladders on A. nodosum serve as a flotation device, which allows sunlight to reach the plant better, aiding photosynthesis.

Excess sperm can be released during the reproduction of Ascophyllum nodosum, which can then act as a food source for plankton consumers. The coverage created by mats of A. nodosum can serve as protection for several marine species, including barnacles (Semibalanus balanoides), periwinkles (genus Littorina), and marine isopods.