Adenine (often abbreviated to A, chemical formula C5H5N5) is a nitrogenous nucleobase, one of the four commonly found in DNA. When forming part of a deoxynucleotide, the compound is called deoxyadenosine monophosphate (dAMP), and it is attached to a growing DNA chain by addition of a deoxyadenosine triphosphate (dATP) precursor. It may also be found in RNA where its monomeric constituent is called simply adenosine monophosphate (AMP) from a adenosine triphosphate (ATP) precursor.

Adenine is a purine derivative, and it base-pairs with thymine in DNA or uracil in RNA, in both cases using two hydrogen bonds. It may sometimes be found methylated in eukaryotic genomes, where the sixth nitrogen on the purine ring has a methyl group attached to form N6-methyladenine.