Radioactive wastewater contains high concentrations of ⁹⁰Sr (strontium-90), which poses a serious threat to human health and ecosystems. Layered ammoniated zirconium phosphate (A-ZrP) was synthesized herein using a one-pot hydrothermal method for the adsorptive removal of Sr²⁺. ZrP was ammoniated between its structured layers to increase interlayer spacing and facilitate the access of Sr²⁺ ions to adsorption sites and realize adequate ion exchange. A-ZrP achieved a faster equilibration time (120–240 min) compared with that of other ZrPs (250–500 min). A-ZrP exhibited excellent Sr²⁺ uptake selectivity (K_d = 5.3 × 10⁵ mL g⁻¹) and maximum adsorption capacity (q_m = 341 mg g⁻¹) at a pH of 7. Moreover, A-ZrP displayed high selectivity in various contaminated solutions, with a high K_d value of 681 mL g⁻¹, even in artificial seawater. The adsorption mechanism was found to involve ion exchange between NH₄⁺ and Sr²⁺, primarily because of electrostatic interactions and chemical hardness. These results highlight the potential of A-ZrP as an efficient adsorbent for the selective removal of Sr²⁺ ions from wastewater.