Pterin is a heterocyclic compound composed of a pteridine ring system, with a keto group and an amino group on positions 4 and 2 respectively. Several tautomers of pterin exist and are shown below. Derivatives of pterin include pterins and folates.

Pterins, as a group, are compounds that are derivatives of the basic pterin structure, described in the beginning, with additional functional groups attached to the pyrazine subring. Pterins were first discovered in the pigments of butterfly wings (hence the origin of their name, from the Greek pteron (πτερόν), wing) and perform many roles in coloration in the biological world. Pterins also function as cofactors in enzyme catalysis.