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

Crabs and mussels, prehistoric bugs, bits of seaweed, tiny minnows, they all get deposited, then stranded, in the cracks and crevices on top of these massive rocks. — Literary Hub, 2 Nov. 2026

Since taurine doesn't deplete with age, adding more of it to your diet – via sources like seafood, dark meats and seaweed – isn't likely going to counter inflammaging. — New Atlas, 1 Nov. 2026

To protect Mexico’s tourism-reliant economy, the Navy is fighting a massive sargassum bloom that dumps thousands of tons of seaweed onto Caribbean beaches daily. — David Shortell Cristopher Rogel Blanquet, New York Times, 28 Oct. 2026

The arrangement does not grant Indigenous peoples ownership of the sea or seabed, but protects the area’s ancestral uses, including fishing, shellfish and seaweed gathering, as well as access to sacred sites and areas used for navigation. — ABC News, 28 Oct. 2026

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)