90Sr2+remediation from acidic nuclear waste solution and contaminated seawaterattracts great public attention but is very challenging because the removal performance toward soft Sr2+ by traditional oxidic ion-exchange materials are greatly affected by hard cations including H+, Na+, and Ca2+. We report a layered oxidic coordination polymer [(CH3)2NH2][ZrCH2(PO3)2F] (SZ-4) showing a notable advance over existing ion-exchange materials in 90Sr removal efficiencies with elevated distribution coefficients and record-high uptake capacities under acidic conditions. Moreover, the removal of Sr2+ is not significantly affected by the presence of a large excess of hard cations, leading to the successful cleanup of 90Sr-contaminated seawater samples. Such superior capabilities are directly visualized by well-refined single-crystal structures during in situ Sr2+sorption process, revealing a distinctive two-step intercalation mechani** and a unique soft cation uptake selectivity achieved through collaborative coordination by the host layer and the interlayer dimethylamine (DMA) as a soft N-donor ligand.