Guanosine is a molecule (known as a nucleoside) that is formed when guanine is attached to a ribose ring (also known as a ribofuranose) via a β-N₉-glycosidic bond.

Guanosine can be phosphorylated to become GDP (guanosine diphosphate) and GTP (guanosine triphosphate) among other things.

If guanine is attached to a deoxyribose ring, it is known as a deoxyguanosine.