This work studied to use paper waste sludge for hydrochar production by hydrothermal carbonization with supporting NaOH solution at carbonization temperature of 200 °C for ammonium adsorption. The obtained original hydrochar was then modified with FeCl3 to enhance the ammonium adsorption capacity. The batch adsorption experiments were conducted to test the effects of solution pH, contact time and initial ammonium concentration. The properties of original paper waste sludge hydrochar (OPSH) and iron modified paper waste sludge hydrochar (FPSH) were investigated using scanning electron microscopy (SEM, EDS), Powder X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR). The results show that iron ions were successfully loaded on the OPSH with the most optimum adsorption capacity of ammonium at iron impregnation ratio of 10%(w/w). The results illustrate that the suitable conditions for ammonium adsorption were pH of 9, 120 min of contact time and an initial ammonium concentration of 80 mg/L. The highest adsorption capacities were 8.62 mg/g and 11.55 mg/g for OPSH and FPSH-10, respectively. The experimental data was fitted well by Langmuir, Sips models and the pseudo-second-order model with the R2 of 0.9797 and 0.9946 (Langmuir model), 0.9926 and 0.9958 (Sips model) for OPSH and FPSH, respectively. In addition, the R2 of the pseudo-second-order model were 0.9943 and 0.9881 for OPSH and FPSH, respectively. The adsorption process was controlled by monolayer chemisorption mechanism through cation exchange, surface complexation and electrostatic attraction.