Precise monitoring of changes in ion concentration in electrolytic environments is of growing interest in multiple fields, such as bioelectronics, food packaging, agricultural sensing, and control of industrial chemical processes. However, combining sensitivity, ion-selectivity, and cost reduction has been proven to be a difficult task. In this work, we use an organic mixed ionic–electronic conductor [poly(3,4-ethlyenedioxythiophene) doped with poly(styrene sulfonate), PEDOT:PSS] to realize a sensor showing good selectivity and sensitivity to alkali ions without employing ion-selective membranes. We achieve this by combining a straightforward impedance analysis and static current–voltage measurement of an organic electrochemical transistor. We show that, after a calibration stage, the composition of unknown solutions can be determined. The ease of fabrication of this system, combined with the proposed measurement method and the potential biocompatibility of the organic semiconductor, makes such a sensor suitable for applications in biological environments, such as within the body or soil.