Thymidine is a molecule (known as a nucleoside) that is formed when thymine is attached to a deoxyribose ring (also known as a deoxyribofuranose) via a β-N1-glycosidic bond.

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