Guanine (often abbreviated to G, chemical formula C5H5N5O) is a nitrogenous nucleobase, one of the four commonly found in DNA. When forming part of a deoxynucleotide, the compound is called deoxyguanosine monophosphate (dGMP), and it is attached to a growing DNA chain by addition of a deoxyguanosine triphosphate (dGTP) precursor. It may also be found in RNA where its monomeric constituent is called simply guanosine monophosphate (GMP) from a guanosine triphosphate (GTP) precursor.

Guanine is a purine derivative, and it base-pairs with cytosine using three hydrogen bonds.