<p>The swimming crab, <i>Portunus trituberculatus,</i> supports the world’s largest crab fishery. Hard-shell crabs have fully developed exoskeletons and are rich in meat, while soft-shell crabs, which have recently molted, contain less meat. To determine their nutritional status and feeding behavior, we measured amino acids (AAs), organic carbon (OC), δ¹³C, δ¹⁵N, and ²¹⁰Po in the muscle of hard- and soft-shell crabs collected from the eastern Yellow Sea in May 2024. The concentrations of total AAs (TAAs) and OC were approximately 1.4 and 1.3 times higher, respectively, in hard-shell crabs than in soft-shell crabs. A significant positive correlation between TAAs and OC (r² = 0.74, <i>p</i> &lt; 0.05) suggests that hard-shell crabs have a better nutritional status, due to the consumption of higher-quality food. Hard-shell crabs also exhibited significantly higher δ¹³C values and ²¹⁰Po activities than soft-shell crabs, whereas δ¹⁵N values showed no significant difference between the two groups. These results indicate that hard-shell crabs primarily consume higher trophic-level prey compared with soft-shell crabs, despite both occupying the same trophic position. Thus, this difference in food sources may be influenced by post-molt hiding behavior in soft-shell crabs, which limits access to high-trophic prey, or alternatively, by competition for food in coastal waters.</p>