Phycocyanin is a pigment from the light-harvesting phycobiliprotein family, along with allophycocyanin and phycoerythrin. It is an accessory pigment to chlorophyll. All phycobiliproteins are water soluble and therefore cannot exist within the membrane like carotenoids, but aggregate forming clusters that adhere to the membrane called phycobilisomes. Phycocyanin absorbs orange and red light, particularly near 620 nm (depending on which specific type it is), and emits fluorescence at about 650 nm (also depending on which type it is). Allophycocyanin absorbs and emits at longer wavelengths than Phycocyanin C or Phycocyanin R. Phycocyanins are found in Cyanobacteria (previously called blue-green algae). Phycobiliproteins have fluorescent properties that are used in immunoassay kits. Phycocyanin is from the Greek phyco meaning “algae” and cyanin is from the English word “cyan", which is derived from the Greek “kyanos" and means blue-green. The product Phycocyanin, produced by Spirulina, is used in the food and beverage industry as a natural colouring agent 'Lina Blue' and is found in sweets and ice cream.

The phycobiliproteins are made of subunits having a protein backbone to which a linear tetrapyrrole chromophores are bound.