Cytosine (often abbreviated to C, chemical formula C4H5N3O) is a nitrogenous nucleobase, one of the four commonly found in DNA . When forming part of a deoxynucleotide, the compound is called deoxycytidine monophosphate (dCMP), and it is attached to a growing DNA chain by addition of a deoxycytidine triphosphate (dCTP) precursor. It may also be found in RNA where its monomeric constituent is called simply cytidine monophosphate (CMP) from a cytidine triphosphate (CTP) precursor.

Cytosine is a pyrimidine derivative, and it base-pairs with guanine using three hydrogen bonds. Cytosine is often found methylated in eukaryotic genomes to form 5'-methylcytosine.