A bryozoan is any member of the phylum Bryozoa. Bryozoans (the name means "moss life"), also known as polyzoans, are tiny animals, growing to about 1 mm (0.05 in) in length, which live in colonies of many polyps in seawater and occasionally in fresh or brackish water. They have ciliated tentacles and a hard, box-like, calcium carbonate skeleton. Up to 2 million individuals, called zooids, may be linked to form lacy patterns that occur on the surface of seaweeds. There are nearly 4,000 living species.

Bryozoans first appeared in the early Ordovician period, about 400 million years ago. Fossil bryozoans are very plentiful and important in the rock-forming process. Archimedes was an ancient corkscrew-shaped bryozoan.