Astaxanthin is a naturally occurring xanthophyll. It gives the tissues of fish, such as salmon or trout, as well as the shells of crustaceans a characteristic pink-red colour. Animals and people do not have the ability to synthesize astaxanthin de novo. Some algae, yeast, bacteria, and plants possess this ability. Owing to its chemical structure, astaxanthin is characterized by strong antioxidant properties. It is considered one of the most powerful natural antioxidants. Scientific studies carried out so far have proven that astaxanthin, if taken in large doses, does not exhibit any prooxidative and toxic properties. Hence, astaxanthin is used as an ingredient in dietary supplements and, moreover, as a feed additive for fish and, to a lesser extent, for poultry. More and more frequently, it is also applied as a functional food component. The paper presents a detailed characterization of astaxanthin in terms of its chemical structure, occurrence, application safety, and potential applications thereof to produce functional food.