Abstract <p>In the current study, a potentiometric all-solid-state composite ion-selective electrode for the determination of iodide ions is described. The synthesized salicylaldehyde thiosemicarbazone-mercury(II) complex (<b>SATSC-Hg(II)</b>) (1 : 1) was applied as the electroactive material in the electrode structure. Various composite ion-selective electrodes were prepared with different compositions, including SATSC-Hg(II), graphite, multi-walled carbon nanotubes (<b>MWCNTs</b>), ionic additives, and some mediators (various plasticizers or paraffin oil) to identify the most effective composite sensing layer composition exhibiting satisfactory potentiometric characteristics. The sensing layer composition consisting of 10 mg SATSC-Hg(II) complex, 26 mg dioctyl phthalate plasticizer, 5 mg MWCNTs, 4 mg methyltrioctylammonium chloride, and 65 mg graphite yielded the most favorable potentiometric performance properties. The electrode demonstrated a linear operating range of 1.0 × 10^–5 to 0.1 M, a slope value of 55.2 ± 0.2 mV/decade, a detection limit of (9.0 ± 0.2) × 10^–6 M, a pH operating range of 4–8, and a response time of approximately 60 s. The proposed electrode also displayed reproducible, stable, and selective potentiometric response to iodide ions. The analytical application of the electrode involved its use as an indicator electrode in the titration of iodide ions in synthetic solutions and for the determination of KI content in a pharmaceutical used for the treatment of goiter by the multiple standard addition method. Compared to polymer membrane electrodes, the renewability and reusability of the sensing surface of the proposed electrode, its suitability for miniaturization, and its robust structure offer significant advantages.</p>