Spirulina (Arthrospira spp.) is a blue-green microalga within the phylum Cyanophyta, also known as Cyanobacteria. It has superior nutritional value, clinically proven health benefits, and beneficial interaction in fermentation and preservation processes. Therefore, Spirulina is a functional ingredient and a key ingredient in functional foods for particular dietary use. Nutritional facts show that Spirulina has over 60% of protein, with relatively low fat and sugar contents. The clinical studies have highlighted that health benefits of Spirulina are caused by its high antioxidant activity, especially due to the presence of a unique blue pigment-protein complex—phycocyanin. Contrary to other edible algae, such as Chlorella, Spirulina cell membrane is easy to be digested, and nutrients have high bioavailability. Spirulina as a potential source for vitamin B12 is confirmed. The present chapter discusses some examples of cultivation parameters’ affecting Spirulina’s nutritional value. In addition, the most significant producers, mainly in Asia, with market share of powder, tablets, and capsules, are summarized. Fresh frozen Spirulina is a new, rapidly growing trend in Western countries due to its milder taste. Spirulina-containing products in the market are presented, and many commercial products of snacks, drinks, pasta, and dairy products are given. Studies on Spirulina as a functional ingredient that show its potential to boost fermentation process and prevent pathogen microbe development are summarized.