What Is Seaweed? Kelp, Algae and What Washes Up on the Beach
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How we wrote this: A plain-English summary of NOAA's National Ocean Service page "What is seaweed?", checked October 29, 2026. This guide is informational, not health or diet advice.
Plants and algae, big and small
"Seaweed" is the common name for countless species of marine plants and algae. They grow in the ocean and also in rivers, lakes and other water. NOAA describes a wide range:
- Microscopic seaweeds, such as phytoplankton, drift in the water and form the base of most marine food chains.
- Giant kelp grows in underwater "forests" that tower like redwoods from the seafloor.
- Most are medium-sized, come in red, green, brown and black, and wash up on beaches nearly everywhere.
Not really a weed
NOAA calls the name a misnomer. A weed spreads so much that it harms its habitat, but seaweeds are essential food and habitat for countless sea creatures.
How people use it
- Food: nori, the wrap on sushi rolls, has been used in Japan for at least 1,500 years. Seaweed is rich in vitamins, minerals and fiber.
- Products: seaweeds act as binding agents (emulsifiers) in goods like toothpaste and fruit jelly, and as softeners in cosmetics and skin care.
- Research: some contain anti-inflammatory and anti-microbial compounds that researchers are studying.
At the beach
Washed-up seaweed is a natural part of the shoreline. Leave it where you find it unless local rules say otherwise. For foam that sometimes forms from decaying algae, see what is sea foam.
Related: why the ocean is blue, beach cleanups.