Kelp refers to several species of large, brown algae that can grow to enormous sizes far out in the depths of the ocean. Kelp is a type of seaweed, but not all seaweed is kelp: "seaweed" loosely describes any type of vegetation growing in the ocean, including many other types of algae and plants.

Kelp is a regular part of a normal human diet in many parts of the world, such as Japan, Alaska, and Hawaii. It is also incorporated into some vitamin and mineral supplements because of its nutrient value. Kelp is a good source of folic acid (a B vitamin), as well as many other vitamins and minerals—especially iodine; but iodine is also a potential source of side effects (see Safety Issues below).

Supplements containing kelp can be purchased at most pharmacies and health food stores. Kelp used in food preparation is available at groceries that stock specialties for Asian cooking.