Iron deficiency is a serious agricultural problem, particularly in alkaline soils. Secretion of coumarins by Arabidopsis thaliana roots is induced under iron deficiency. An essential enzyme for the biosynthesis of the major Arabidopsis coumarins, scopoletin and its derivatives, is Feruloyl-CoA 6'-Hydroxylase1 (F6'H1), which belongs to a large enzyme family of the 2-oxoglutarate and Fe2+-dependent dioxygenases. We have functionally characterized another enzyme of this family, which is a close homologue of F6'H1 and is encoded by a strongly iron-responsive gene, At3g12900. We purified At3g12900 protein heterologously expressed in Escherichia coli and demonstrated that it is involved in the conversion of scopoletin into fraxetin, via hydroxylation at the C8 position, and that it thus functions as a scopoletin 8-hydroxylase (S8H). Its function in plant cells was confirmed by the transient expression of S8H protein in Nicotiana benthamiana leaves, followed by metabolite profiling and biochemical and ionomic characterization of Arabidopsis s8h knockout lines grown under various iron regimes. Our results indicate that S8H is involved in coumarin biosynthesis, as part of mechanisms used by plants to assimilate iron.