We report on a ‘green’ method for the utilization of carbon dioxide as C1 unit for the regioselective synthesis of (E)‐cinnamic acids via regioselective enzymatic carboxylation of para‐hydroxystyrenes. Phenolic acid decarboxylases from bacterial sources catalyzed the β‐carboxylation of para‐hydroxystyrene derivatives with excellent regio‐ and (E/Z)‐stereoselectivity by exclusively acting at the β‐carbon atom of the C=C side chain to furnish the corresponding (E)‐cinnamic acid derivatives in up to 40% conversion at the expense of bicarbonate as carbon dioxide source. Studies on the substrate scope of this strategy are presented and a catalytic mechanism is proposed based on molecular modelling studies supported by mutagenesis of amino acid residues in the active site.