In this study, a fast, reproducible and simple-to-apply a new potentiometric determination method was developed for the evaluation of iron (III) reducing power (FRAP), one of the antioxidant capacity tests. For this purpose, an all-solid-state contact iron (III)-selective poly vinyl chloride (PVC) membrane electrode (FRAP-PME) has been developed. In the structure of the developed polymeric membrane electrode (PME), Fe(II)-quercetin compound was used as the active ingredient. It was obtained that PME exhibits a linear potentiometric behavior against the concentration change of Fe(III) ion between 1.0 × 10−1–1.0 × 10−5 mol L−1 and the detection limit is 8.0 × 10−6 mol L−1. PME showed a very rapid potential response (40–45 s) and was found to have a very high selectivity towards Fe(III) ion in the presence of other species that might interfere. The proposed potentiometric method has been successfully applied to evaluate the iron (III) reducing power of plant extracts.