Spirulina is a genus of the phylum Cyanobacteria. Cyanobacteria are classified as either blue green algae or as blue green bacteria. Spirulina is a popular food supplement in Japan and is marketed as a nutritional supplement in the United States.

Spirulina, wheat grass, barley grass and chlorella are sometimes referred to as green foods. There are several species of spirulina. The ones most commonly used in nutritional supplements are Spirulina platensis.