<p>Muddy tidal flats (hereafter referred to as mudflats) dominate China’s coastal tidal wetlands. A comprehensive assessment of their carbon stock characteristics is thus essential for refining the understanding of China’s marine carbon sink capacity. In this study, sediment core sampling was conducted to systematically analyze sedimentary organic carbon (SOC) content, grain-size parameters, clay mineral composition, and carbon stock characteristics of mudflat sediments from different bays in Zhejiang Province. The results indicate that muddy sediments predominate in all three studied bays. The semi-enclosed Yanpu Bay (YP Bay), with stable hydrodynamics and small-river input, has significantly higher SOC content (0.63%±0.04%) than the high-energy Hangzhou Bay (HZ Bay) (0.55%±0.07%, large-river input) and the open Aiwan Bay (AW Bay) (0.36%±0.07%, no river input). Within individual bays, hydrodynamic conditions varied little across tidal zones, and no statistically significant differences in SOC content were detected. Nevertheless, SOC levels were generally higher in the high-tidal zone than in the middle-tidal and low-tidal zones. Analyses of total organic carbon to total nitrogen ratios (C/N) and stable carbon isotopes (δ¹³C) suggest that SOC in all three bays originates from mixed marine and terrestrial sources. Terrestrial organic matter contributes more prominently in HZ Bay and AW Bay due to the influence of Changjiang (Yangtze) River runoff, whereas YP Bay is dominated by marine-derived organic matter. By integrating data from mudflats along the Chinese coast, this study further reveals that carbon stock ranges from 18.2 to 141.5 Mg C/ha, with a total carbon storage of approximately 51.9 Tg C. The SOC stock of mudflats exhibits a “high-medium-high” spatial distribution pattern from north to south, where climate, riverine transport, and sedimentary environments are likely the primary factors driving this variation.</p>