any aquatic invertebrate animal having a shell and belonging to the phylum Mollusca, the class Crustacea (phylum Arthropoda), or the phylum Echinodermata. The term is often used for the edible species of the groups, especially those that are fished or raised commercially.

Bivalve mollusks, including oysters, mussels, scallops, and clams, rank among the most commercially important shellfish throughout the world. Certain gastropod mollusks, such as abalone, whelk, and conch, are also marketed. The main crustacean forms caught and eaten are the shrimp and prawns of the genera Crangon and Palaemon off the coast of Europe and the genus Penaeus in the coastal waters from North Carolina to Mexico. The American lobster, the Norway lobster (also called Dublin Bay prawn), and the South African rock lobster are highly valued, as are the king crab and the Dungeness crab and its related species. Among echinoderms, sea urchins and sea cucumber (trepang, or bêche-de-mer) are locally popular. There is considerable confusion over the nomenclature of shellfish, compounded by the restaurateur's tendency to name his offerings based on size rather than species.

After being harvested, all shellfish are highly perishable. Many types are cooked live to protect the consumer against the effects of spoilage. Most shellfish benefit from brief and gentle cooking; with high heat they may disintegrate or turn rubbery, and the flavour becomes disagreeably strong. Conch and abalone, however, must be pounded to tenderize the tough meat. Shellfish are often served with rich or highly seasoned sauces.