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

The Coast & Culture trail includes a historical tour of Victoria’s Inner Harbor with a First Nations guide, as well as a coastal tour exploring the varieties of seaweeds that live just offshore. —Carolyn Heller, Forbes.com, 22 Jan. 2026

British chef Andrew Gaskin’s multi-course menus, featuring scallops and peas in pumpkin oil and lobster bisque with seaweed Johnny cakes, are full of Caribbean flavor, but light enough to keep anyone planning to join tomorrow’s sunrise-yoga class happy. —Alex Postman, Condé Nast Traveler, 22 Jan. 2026

Other edible examples include some seaweeds, and leafy greens such as Malabar spinach, native to Asia, and molokhia, popular in parts of Africa and the Middle East. —Andrew Coletti, Popular Science, 14 Jan. 2026

Cyanobacteria in the sea weed can convert that atmospheric nitrogen into a form that seaweed can be used for growth. —Bill Kearney, Sun Sentinel, 14 Jan. 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)