- Kingdom: Plantae
- Phylum: Ochrophyta
- Class: Ulvophyceae
- Order: Fucales
- Family: Fucaceae
- Scientific name: Ascophyllum nodosum
- Norwegian: grisetang
This a large brown seaweed, very common along the Norwegian coastline. It is known as rockweed, Norwegian kelp, knotted kelp or knotted wrack. The stipe is irregularly branched with egg-shaped air bladders. They may reach a length of 2 meters. The color is olive-green.
It is most common in sheltered and moderate exposed locations, where it may dominate the tidal zone.
This is originally an European species, common in the North East Atlantic Ocean. Registration has been done along the Pacific coast of North America, where it is regarded as en invasive species.