The need to treat radioactive aqueous waste has increased, and decontamination of such waste is of high priority. This study demonstrates a method of fabricating a new composite membrane to effectively remove Cs+ and Sr2+ ions using KTS-3, a metal sulfide ion exchanger. The study utilized an electrospinning method to fabricate polyacrylonitrile (PAN) nanofiber support to immobilize the adsorbent KTS-3. Through pulverization using ball milling, KTS-3 was dispersed evenly and fixed to the PAN support which offered high water-permeability and large surface area. The ratio of KTS-3 to the polymer support was optimized to maximize the adsorption performance of the composite membrane. Consequently, we obtained high maximum adsorption capacities of 133.6 and 32.4 mg g−1 for Cs+ and Sr2+, respectively. Moreover, the high adsorption capacity was kept in the filtration test when the flux reaches 2000 L m−2 h−1. The newly fabricated material provides an attractive strategy to utilize adsorbents effectively for the continuous selective removal of radionuclides from contaminated wastewater.