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The emerging field of blue carbon science is seeking cost-effective ways to estimate the organic carbon content of soils that are bound by coastal vegetated ecosystems. Organic carbon (C_org) content in terrestrial soils and marine sediments has been correlated with mud content (i.e., silt and clay, particle sizes &lt; 63 µm), however, empirical tests of this theory are lacking for coastal vegetated ecosystems. Here, we compiled data (<i>n</i> =  1345) on the relationship between C_org and mud contents in seagrass ecosystems (79 cores) and adjacent bare sediments (21 cores) to address whether mud can be used to predict soil C_org content. We also combined these data with the <i>δ</i>¹³C signatures of the soil C_org to understand the sources of C_org stores. The results showed that mud is positively correlated with soil C_org content only when the contribution of seagrass-derived C_org to the sedimentary C_org pool is relatively low, such as in small and fast-growing meadows of the genera <i>Zostera</i>, <i>Halodule</i> and <i>Halophila</i>, and in bare sediments adjacent to seagrass ecosystems. In large and long-living seagrass meadows of the genera <i>Posidonia</i> and <i>Amphibolis</i> there was a lack of, or poor relationship between mud and soil C_org content, related to a higher contribution of seagrass-derived C_org to the sedimentary C_org pool in these meadows. The relatively high soil C_org contents with relatively low mud contents (e.g., mud-C_org saturation) in bare sediments and <i>Zostera</i>, <i>Halodule</i> and <i>Halophila</i> meadows was related to significant allochthonous inputs of terrestrial organic matter, while higher contribution of seagrass detritus in <i>Amphibolis</i> and <i>Posidonia</i> meadows disrupted the correlation expected between soil C_org and mud contents. This study shows that mud is not a universal proxy for blue carbon content in seagrass ecosystems, and therefore should not be applied generally across all seagrass habitats. Mud content can only be used as a proxy to estimate soil C_org content for scaling up purposes when opportunistic and/or low biomass seagrass species (i.e., <i>Zostera</i>, <i>Halodule</i> and <i>Halophila</i>) are present (explaining 34 to 91 % of variability), and in bare sediments (explaining 78 % of the variability). The results obtained could enable robust scaling up exercises at a low cost as part of blue carbon stock assessments.