Flavins (from Latin flavus, "yellow") are organic compounds, like their base, pteridine. They are formed by the tricyclic heterocycle isoalloxazine. The biochemical source is the vitamin riboflavin. The flavin moiety is often attached with an adenosine diphosphate to form flavin adenine dinucleotide (FAD), and, in other circumstances, is found as flavin mononucleotide (or FMN), a phosphorylated form of riboflavin. It is in one or the other of these forms that flavin is present as a prosthetic group in flavoproteins.

The flavin group is capable of undergoing oxidation-reduction reactions, and can accept either one electron in a two-step process or two electrons at once. Reduction is made with the addition of hydrogen atoms to specific nitrogen atoms on the isoalloxazine ring system:

In aqueous solution, flavins are yellow-coloured when oxidized, taking a red colour in the semi-reduced anionic state or blue in the neutral (semiquinone) state, and colourless when totally reduced. The oxidized and reduced forms are in fast equilibrium with the semiquinone (radical) form, shifted against the formation of the radical:

Fl_ox + Fl_redH₂ ⇌ FlH^•

where Fl_ox is the oxidized flavin, Fl_redH₂ the reduced flavin (upon addition of two hydrogen atoms) and FlH^• the semiquinone form (addition of one hydrogen atom).

In the form of FADH₂, it is one of the cofactors that can transfer electrons to the electron transfer chain.