Jinhae–Masan Bay is a semi-enclosed coastal embayment where strong summer stratification and eutrophication frequently promote harmful algal blooms and bottom-water hypoxia. In this study, seawater samples were collected from the inner bay to the outer reference site. We examined concentration and radiocarbon isotope of Dissolved Organic Carbon (DOC) to investigate the behavior and characteristics of DOC under summer hypoxia-affected conditions. DOC concentrations were generally high at the surface, suggesting potential accumulation associated with enhanced biological production, and slow water exchange. High DOC concentrations near the bottom at some stations appear to be linked to organic matter remineralization under hypoxic conditions, sediment-derived inputs, or prolonged residence of bottom-water mass. Radiocarbon isotope data will also be examined to better understand DOC cycling.