Developing efficient, selective, and eco-friendly adsorbents for Cs+ treatment is crucial for ensuring the environmental responsibility of nuclear energy and mitigating the environmental impact of radioactive waste. This study introduced a novel strategy involving carboxylation and metal-mediated crosslinking to enable in situ formation and stabilization of Prussian blue (PB) onto the lignin matrix. PB-immobilized lignin (PB-Lig) exhibited a robust structure with abundant PB nanocubes, facilitating highly rapid and effective Cs+ adsorption. PB-Lig demonstrated applicability across different pH levels and displayed outstanding specificity toward Cs+, making it suitable for diverse conditions. It achieved equilibrium adsorption capacity within 480 s, removing up to 166.5 mg/g of Cs+. Furthermore, PB-Lig showed remarkable stability in an aquatic environment without PB leaching, even after repeated adsorption-desorption cycles. PB-Lig captured Cs+ through strong chemical interactions and lattice trapping facilitated by the exchange of K+ within the PB structure. In summary, PB-Lig presents a sustainable and practical solution for radioactive Cs+ remediation, highlighting new opportunities for biomass utilization in environmental applications. Notably, PB-Lig is the first reported lignin-based Cs+ adsorbent.