Polymers can exhibit adsorbent behavior and are used to remove pollutants from environments. Their adsorption properties can be tailored to capture specific contaminants effectively. In this research, an ammonium-based ion exchange polymer was developed using the one-pot approach method. It allows for simultaneous polymerization and functionalization, reducing the number of reaction steps and purification processes. The developed ammonium-based polymer is a novelty material for removing phosphate ions from an aqueous solution. Several essential factors (temperature, contact time, initial phosphate concentration, and adsorbent dosage) that affect the removal process of phosphate ions during adsorption were investigated. This research aims to develop a novel organic polymer using the one-pot approach for removing phosphate from aqueous solutions and performing adsorption behavior based on isotherm studies. The characterization of ammonium-based polymer was carried out using Fourier Transform Infrared (FTIR) and Scanning Electron Microscopy (SEM), which offered proof of chemical and physical adsorption. The optimum condition during phosphate adsorption using a developed ammonium-based polymer was at a temperature of 25°C, contact time of 20 minutes, initial phosphate concentration of 30 ppm, and adsorbent dosage of 0.05 g, which achieved adsorption capacity (Qe) of 2.99 mg/g and phosphate ion removal percentage of 99.73%. The Langmuir model describes the best adsorption isotherm of phosphate ions on the adsorbent with coefficient value (R2=0.9974). In the future, it can offer an alternative method for treating ion phosphate in aqueous solution or wastewater treatment.