Callinectes sapidus (from the Greek calli- = "beautiful", nectes = "swimmer", and Latin sapidus = "savory"), the Chesapeake or Atlantic blue crab, is a crustacean found in the waters of the western Atlantic Ocean, the Pacific coast of Central America and the Gulf of Mexico. On the Pacific coast of Central America it is largely ignored as a food source as picking the meat is considered too difficult. It is the Maryland state crustacean and the subject of an extensive fishery.

The blue crab is native to the western edge of the Atlantic Ocean from Nova Scotia to Argentina and around the entire coast of the Gulf of Mexico. It has been introduced (via ballast water) to Japanese and European waters, and has been observed in the Baltic Sea, North Sea, Mediterranean Sea and Black Sea.

The natural predators of the blue crab include eels, drum, striped bass, spot, trout, some sharks, humans, and cownose sting rays. The blue crab is an omnivore, eating both plants and animals. Blue crabs typically consume thin-shelled bivalves, annelids, small fish, plants and nearly any other item they can find, including carrion, other blue crabs and human waste.