This work presents a flexible miniaturized ion-selective sensor consisting of a solid-contact reference electrode (RE) and ion-selective electrodes (ISE) fabricated on the same foil. The flexible ion sensor was based on AgCl electrodes screen printed on polyethylene terephthalate foil. The RE was formed by casting a mixture of plasticized polyvinylchloride (PVC) and KCl on the AgCl electrode. The ISE consisted of a stack of AgCl electrode, cellulose gel layer loaded with 0.1 M KCl, and plasticized PVC based ion-selective membrane containing ionophores. The flexible ion sensor showed a sensitivity close to the Nernstian value in a biological relevant range.