The expansion of nuclear power generation in East Asia, together with the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident, has increased concern over radionuclides in the marine environment. Among these radionuclides, ¹³⁷Cs is of particular concern because of its relatively long half-life (30.17 years) and its chemical similarity to potassium, which may facilitate bioaccumulation in marine organisms and trophic transfer through marine food webs. However, previous studies have focused primarily on commercially important fisheries resources, and information on the accumulation of radiocesium in diverse marine organisms remains limited. In this study, surface seawater, sediments, and marine organisms representing various trophic levels were collected from two coastal stations around Jeju Island, Korea, in 2024 and 2025 to evaluate the bioaccumulation characteristics of radiocesium. Because ¹³⁷Cs activity concentrations in the environment are generally low, stable cesium (¹³³Cs) was also analyzed as a complementary proxy to support the interpretation of ¹³⁷Cs bioaccumulation. Mercury (Hg), which is known to biomagnifying in marine food webs, was also measured to compare its trophic transfer characteristics with those of radiocesium. Carbon and nitrogen stable isotope ratios (δ¹³C and δ¹⁵N), together with nitrogen isotope ratios of amino acids analysis, were used to identify food sources and estimate trophic positions of marine organisms. The concentration factor (CF) was calculated to evaluate species-specific cesium accumulation, and the trophic magnification factor (TMF) was estimated based on the relationship between contaminant concentrations and trophic position to assess trophic transfer through the marine food web. In addition, ¹³⁷Cs/¹³³Cs ratios in seawater and marine organisms were compared to evaluate the relative accumulation of radiocesium in marine organisms. This study provides baseline information for understanding the ecological behavior of radiocesium in the coastal ecosystem of Jeju Island, Korea. Furthermore, a multi-tracer approach integrating radiocesium, stable cesium, mercury, bulk stable isotopes, and amino acid-specific nitrogen isotopes provides a more comprehensive evaluation of ¹³⁷Cs bioaccumulation and trophic transfer.