Ammonium (NH4 ⁺) is a main constituent of landfill leachates (50–2200 mg L ⁻¹) which has to be removed prior to indirect (<200 mg L ⁻¹) or direct discharge (<10 mg L ⁻¹) during landfill operation and aftercare, i.e., for more than 100 years after the end of waste disposal. In this study, lab-scale experiments regarding the sorption of NH4 ⁺ from landfill leachates using natural and modified clinoptilolite (Ca0.5,Na,K)6(Al6Si30)O72·20H2O) were conducted to assess the applicability of the innovative ion exchanger loop stripping (ILS) process for ammonium recovery. Samples of 20 g clinoptilolite after different pre-treatments (Ca loading, Na loading, natural loading) were shaken with 500 mL of each leachate from ten Austrian landfills (75 mg L ⁻¹ < c(NH4 ⁺) < 2805 mg L ⁻¹; 7.7 < pH < 8.7) for 24 h. Between 13% and 61% of the dissolved NH4 ⁺ was adsorbed to the clinoptilolite, which remained stable during the experiment. In summary, our study indicates that the ILS process is highly promising with respect to NH4 ⁺ recovery from landfill leachates, but further research is needed to reach threshold values for direct discharge.