Ion sensing represents a grand research field with tremendous challenges and ample opportunities. A proper operation of ion sensors demands a robust reference electrode (RE), but on-chip integration of a conventional liquid-filled RE is incompatible with semiconductor technology for manufacturing ion-sensitive field-effect transistors as electronic sensors. Here, we demonstrate a reference-less semiconductor ion sensor, RELESIS, that integrates an interdigitated electrode (IDE) with a field-effect transistor. As a constant solution potential is no longer necessary, the use of RELESIS eliminates the need of any RE in ion sensing. The evaluated IDE comprises two intertwined metallic combs, each being covered with a specific sensing layer. One of the combs is connected to the transistor for readout while another is biased with a voltage signal source. Our extensive measurement results with pH sensing confirm that the sensitivity of RELESIS is exclusively determined by the sensitivity difference of the two sensing layers. By eliminating bulky REs, the RELESIS may find myriad ion-sensing applications owing to its miniaturisability, integrability, flexibility and cost advantages.