A cockle is a small, edible, marine bivalve mollusc. Although many small edible bivalves are loosely called cockles, true cockles are species in the family Cardiidae. True cockles live in sandy, sheltered beaches throughout the world. The distinctive rounded shells are bilaterally symmetrical, and are heart-shaped when viewed from the end. Numerous radial evenly spaced ribs are a feature of the shell in most but not all genera (for an exception, see the genus Laevicardium, the egg cockles, which have very smooth shells).

The common name "cockle" is also given by seafood sellers to a number of other small, edible marine bivalves which have a somewhat similar shape and sculpture, but are in other families such as the Veneridae (Venus clams) and the ark clams. Cockles in the family Cardiidae are sometimes referred to as "true cockles" to distinguish them from these other species.