The development of selective and sensitive analytical methods for ammonium detection is important for the evaluation of the environmental effects of water pollution and the understanding of nitrogen cycling in ecosystems. In the current study, a new Pt nanoparticle microelectrode with high selectivity, sensitivity, accuracy and anti-interference capacity was successfully prepared for the quantitative measurement of ammonia concentration in artificial and real wastewater samples. The prepared Pt nanoparticle microelectrode had a selectively electrochemical catalytic oxidation activity for ammonium. Ammonium oxidation peak current presented a linear correlation with ammonium concentrations in artificial and real wastewater samples. Ammonium response was 92% of the initial value after 120 measurements, indicating that the prepared microelectrode is reliable and stable as an ammonium sensor. The lifetime of the modified microelectrode was of at least 150 days at room temperature and of at least 500 repetitive measurements. The present method is a simple way of preparing an ammonium sensor with very good reproducibility. Moreover, the proposed electrochemical method has been validated by comparing the obtained results of the analysis of ammonium in artificial and real wastewater samples with those obtained from spectrophotometry.