<p>Dissolved organic carbon released from marine macrophytes is an important carbon sequestration pathway, but its recalcitrant fraction remains poorly quantified. Here, we quantified dissolved organic carbon release rates and recalcitrance using empirical data from macroalgal and seagrass species across cold-temperate to subtropical coastal Japan. Dissolved organic carbon release rates ranged from 5 to 462 µmol g-DW⁻¹ d⁻¹, with similar averages between macroalgae and seagrasses. Using degradation dynamics simulated with a reactivity continuum model, we estimated the mean recalcitrant fractions over 100-year timescale as 25% (17–34%, 95% credible interval) for seagrasses and 14% (11–16%) for macroalgae, corresponding to 8% (4–12%) and 4% (3–6%) of annual net primary production, respectively. These regionally constrained estimates indicate that macrophyte-derived recalcitrant dissolved organic carbon generation represents a major carbon storage process comparable to particulate organic carbon storage beyond the source ecosystems.</p><p></p>