Thymine (often abbreviated to T, chemical formula C10H15N2O8) is a nitrogenous nucleobase, one of the four commonly found in DNA . When forming part of a deoxynucleotide, the compound is called deoxythymidine monophosphate (dTMP, or sometimes simply TMP), and it is attached to a growing DNA chain by addition of a deoxythymidine triphosphate (dTTP, TTP) precursor. Thymine is not found in RNA; it is replaced by uracil.

Thymine is a pyrimidine derivative, and it base-pairs with adenine using two hydrogen bonds.