Crinoid, or, any member of a class of colorful, plantlike sea animals related to the starfish. There are two kinds of crinoids: the sea lilies and the feather stars. As soon as they hatch, the sea lilies develop jointed stalks with which they attach themselves for life to an underwater surface, usually a rock. The feather stars also develop stalks for attachment as soon as they hatch, but as they mature, they break free of their stalks to become free-swimming. The crown, or body, of a sea lily is cup-shaped, with many branching, feathery arms that wave to and fro, seizing small sea creatures for food. Mature feather stars have similar crowns, but no stalks.

These animals were much more abundant millions of years ago than they are today. Some kinds of limestone are made up almost entirely of the stems of extinct crinoids. Crinoids are found in warm and cold waters, at depths of 30 feet (9 m) to nearly 18,000 feet (5,500 m).

Crinoids make up the class Crinoidea of the phylum Echinodermata.