Kappaphycus alvarezii (syn. K. cottonii; Eucheuma cottonii) is a species of red algae. It is one of the most important commercial sources of carrageenans, a family of gel-forming, viscosifying polysaccharides. Farming methods affect the character of the carrageenan that can be extracted from the seaweed. This alga grows to two meters long and is green or yellow in color. It is very fast-growing, known to double its biomass in 15 days. This alga is an introduced species and a noxious aquatic weed in Hawaii.