This report discloses the first example of catalytic arylhydroxylation of dehydroalanine with aryldiazonium salts. Aryldiazonium salts, which are generated from aniline precursors under partially aqueous conditions in continuous flow, efficiently react with dehydroalanine in the presence of 10‐15 mol% ferrocene to furnish α−hydroxyarylalanine derivatives (up to 82% yield). The reactions proceed with regioselectivity, broad functional group tolerance, and without polymerization of the dehydroalanine. Furthermore, the products can be used to access α−unnatural amino acids, important targets with application in drug development.