As the risk of radioactive waste generated in large quantities is emerging, rapid and selective cesium ion decontamination has become a significant issue for waste volume reduction. This study demonstrates a novel immobilization strategy to synthesize a composite adsorbent embedding potassium copper hexacyanoferrate (KCuHCF) for effective cesium ion separation. Through an electrospinning technique, a nanofiber matrix was fabricated from a polymer blended with poly-vinyl alcohol and sodium alginate, and in situ immobilization of KCuHCF was achieved via the coordination effect of carboxylate functionality. The immobilized KCuHCF nanoadsorbent content and uniformity were optimized by controlling the ratio of the polymers, which was systematically analyzed through electron microscopy and spectroscopic analyses. The composite adsorbent in the batch experiment exhibited a high adsorption capacity of 114 mg g−1 and selectivity for cesium showing distribution coefficients on the order of 104–105 mL g−1, even under seawater conditions where large amounts of competitive cations coexist. A membrane filtration experiment using the composite as a membrane at a pressure of 2 bar showed efficient decontamination by removing 180 mg cesium per 1 m2 with a high flux of about 148 L m−2 h−1. Thus, the deposition of KCuHCF nanoadsorbent on the surface of the nanofiber and the adsorption filtration method could be a practical alternative in the radioactive wastewater treatment.