Thymine, also known as 5-methyluracil, is a pyrimidine nucleobase. It is found in the nucleic acid DNA. In RNA thymine is replaced with uracil. Thymine can base pair with adenine.

Thymine combined with deoxyribose creates the nucleoside thymidine. Thymidine can be phosphorylated with one, two or three phosphoric acid groups, creating respectively TMP, TDP or TTP (thymidine mono- di- or triphosphate).