Radiotherapy is widely employed in cancer treatment, yet its inevitable side effects, particularly radiation dermatitis (RD), remain a significant concern. Numerous studies focus on alleviating RD symptoms, but approaches to minimize the radiation dose to the skin are underexplored. Shielding skin-damaging photons (10-20 keV) holds promise for skin dose reduction; however, this strategy is hindered by inadequate shielding efficiency of current RD biomaterials and overshielding by conventional high-Z materials which attenuate therapeutic high-energy photons. Here, inspired by the strong absorption of near-threshold-energy photons at the K-edge, an energy-selective shielding strategy by integrating selenium-containing melanin nanoparticles with a carbomer hydrogel is proposed, developing an X-ray filtering skin dressing (SeMNPs@Car). It exhibits up to 96% shielding for low-energy X-ray (10-20 keV) while maintaining over 98% transmission for high-energy photons. In RD animal models, the SeMNPs@Car achieves 100% prevention of ulceration and the longest delay in the onset of RD (approximate to 20 days), outperforming clinical RD treatments. Furthermore, Monte-Carlo-based reconstruction of radiotherapy scenarios demonstrates its clinical applicability. This work incorporates photoelectric absorption properties into hydrogel design, providing valuable insights for RD management and the creation of soft-material-based X-ray filters.