seaweed

noun

sea·​weed ˈsē-ˌwēd 

1

plural seaweeds : any of various aquatic and chiefly marine brown, red, or green algae (such as rockweed, gulfweed, and kelp) that often grow in masses, typically have leaflike blades (see blade entry 1 sense 2e), are usually anchored to a solid substrate (such as a rock) by holdfasts (see holdfast sense 2a), and include some (such as dulse, laver, and sea lettuce) that are used as food

… many seaweeds have floats called pneumatocysts, filled with oxygen or carbon dioxide.

—Michael Le Page

The word seaweed is broad and refers generally to marine macroalgae, which are algae living in salt or brackish water and visible to the eye.

—Elyse Inamine

2

: a mass or growth of seaweeds

… soft crabs were easily visible in the seaweed, because the water was crystal clear then.

—Keith Walters

3

: harvested seaweed or seaweed parts used especially as food, fertilizer, and in cosmetics

… sushi rolls wrapped in thin sheets of dried seaweed …

—Jane Snow

Examples of seaweed in a Sentence

Recent Examples on the Web

Anderson-Haynes noted that other types of seafood and fish, such as seaweed, walnuts, flaxseeds, or chia seeds, are also good dietary sources of omega-3s. —Stephanie Brown, Verywell Health, 3 Dec. 2025

Oil coated patches of sargassum seaweed and the wings of seabirds. —Kylie Williams, Miami Herald, 29 Nov. 2025

Of all the things symbolic of life on the coast, seaweed is among the top. —Ashley MacKin Solomon, San Diego Union-Tribune, 27 Nov. 2025

Rich in iodine, seaweed supports healthy thyroid function, which plays a key role in regulating your metabolism and energy levels, both of which can impact your sleep-wake cycle. —Lauren Manaker, SELF, 26 Nov. 2025

Word History

First Known Use

1577, in the meaning defined at sense 1a

Rhymes for seaweed

• accede
• agreed
• airspeed
• alkyd
• bindweed
• chickweed
• concede
• decreed
• duckweed
• exceed
• flaxseed
• impede

Kids Definition

seaweed

noun

sea·​weed -ˌwēd 

: a plant growing in the sea

especially : a marine alga (as a kelp)