Functional foods are foods that claim to provide health benefits beyond basic nutrition due to certain physiologically active components, which may or may not have been manipulated or modified to enhance their bioactivity. This means a functional food can be enriched with nutrients that may not be naturally inherent to it. Familiar examples include orange juice fortified with calcium or milk fortified with vitamins A and D.

Functional food or medicinal food is any fresh or processed food claimed to have a health-promoting and/or disease-preventing property beyond the basic nutritional function of supplying nutrients. Functional foods are sometimes called nutraceuticals, a name attempting to bring together the words nutrition and pharmaceutical. Nutraceuticals can include food that has been genetically modified. The general category includes processed food made from functional food ingredients, or fortified with health-promoting additives, like "vitamin-enriched" products, and also, fresh foods that have specific claims attached.

The term was first used in Japan in the 1980s, where there is a government approval process, called Foods for Specified Health Use. Underlying the development of functional foods is the concept of "food as medicine".

While the concept of functional foods is relatively new to the United States, other countries such as Japan, Korea, and China have known for ages that certain foods have health benefits. Therefore, while functional foods are considered revolutionary and represent a rapidly growing segment of the food industry, there is a difference between the Western and Eastern perspective. Functional foods have been an integral part of Eastern cultures where they were used for medicinal purposes in traditional Chinese medicine for centuries. From ancient times, the Chinese have used foods for both preventive and therapeutic health effects, a view that is now being increasingly recognized around the world.

What makes a "functional food," however, is its potential ability to positively affect health. Functional foods range from fruits, vegetables, and whole grains, which are naturally high in phytochemicals, to products in which a specific ingredient is added, removed, increased, or decreased. The functional attributes of many traditional foods are continually being discovered, while new food products are being developed with beneficial components.

Examples of functional foods include soy, oats, flaxseed, grape juice, broccoli and other cruciferous vegetables, phytosterol/stanol-enriched margarine, eggs enhanced with omega-3 fatty acids, foods fortified with herbal preparations, and psyllium.