Sulfonated hyper-cross-linked polymers based on 4,4′-bis(chloromethyl)-1,1′-biphenyl (BCMBP) were synthesized via metal-free (SHCP-1) and conventional Lewis acid-catalyzed (SHCP-2) Friedel–Crafts alkylation routes. The sulfonated polymers possessed BET surface areas in excess of 500 m2·g–1. SHCP-1 was investigated for its ability to extract Sr and Cs ions from aqueous solutions via the ion-exchange reaction of the sulfonic acid moiety. Equilibrium uptake data could be accurately modeled by the Dubinin–Radushkevich isotherm, with maximum calculated loading values of 95.6 ± 2.8 mg·g–1 (Sr) and 273 ± 37 mg·g–1 (Cs). Uptake of both target ions was rapid, with pseudo second-order rate constants calculated as 7.71 ± 1.1 (×10–2) for Sr and 0.113 ± 0.014 for Cs. Furthermore, the polymer was found to be highly selective toward the target ions over large excesses of naturally occurring competing metal ions Na, K, Mg, and Ca. We conclude that hyper-cross-linked polymers may offer intrinsic advantages over other adsorbents for the remediation of aqueous Sr and Cs contamination.