Ammonium ion (NH₄⁺) is one of the indicators of water quality; high ammonium concentration [NH₄⁺] in water can cause eutrophication, affect aquatic biota, and cause cell death in the central nervous system of human beings. However, current ion-selective electrodes used for water-quality monitoring are bulky, require frequent calibration owing to membrane fouling, and cannot be integrated into mobile sensor platforms. We fabricated an all-solid-state ion-selective electrode (AS-NISE) for ammonium ion using a two-step process; the first step is electropolymerization deposition on the electrode using a solution of 3,4-ethylenedioxythiophene (EDOT), sodium polystyrene sulfonate (NaPSS), and lithium perchlorate (LiCLO₄), resulting in a solid-state transducer on screen-printed carbon electrodes (SPCEs), and the second is electropolymerization deposition of EDOT, NaPSS, and o-phenylenediamine (o-PD) as ammonium ion–selective membrane (NISM) on top of the transducer. The electropolymerization deposition of the transducer and the NISM were simply achieved by cyclic voltammetry (CV) with potential from 0.0 V to 0.8 V and 50 mVs^-1 scan rates. The fabricated NH₄⁺ISE can detect [NH₄⁺] as low as 5.7×10^-5 M, with slope of 61.9 mV/decade (R²>0.99) and a linear range from 10^-3 M to 1 M. These preliminary results provide an initial insight into the applicability of the simple two-steps fabrication process of NH₄⁺ ISEs for scaling-up purposes, with the ability for miniaturization and integration into a mobile sensor platform.