Guanine is a purine base. Guanine (2-amino-6-oxypurine) is one of five organic bases found in the nucleic acids of cells. It pairs with cytosine in both DNA and RNA. Guanine is named for its occurrence in guano, a natural fertilizer composed mainly of the excrement of seabirds and bats.

Guanosine is a compound containing guanine and the sugar ribose.