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Abstract Coastal wetlands, including seagrass meadows, emergent marshes, mangroves, and temperate tidal swamps, can efficiently sequester and store large quantities of sediment organic carbon (SOC). However, SOC stocks may vary by ecosystem type and along environmental or climate gradients at different scales. Quantifying such variability is needed to improve blue carbon accounting, conservation effectiveness, and restoration planning. We analyzed SOC stocks in 1,284 sediment cores along textgreater6,500 km of the Pacific coast of North America that included large environmental gradients and multiple ecosystem types. Tidal wetlands with woody vegetation (mangroves and swamps) had the highest mean stocks to 1 m depth (357 and 355 Mg ha−1, respectively), 45% higher than marshes (245 Mg ha−1), and more than 500% higher than seagrass (68 Mg ha−1). Unvegetated tideflats, though not often considered a blue carbon ecosystem, had noteworthy stocks (148 Mg ha−1). Stocks increased with tidal elevation and with fine (textless63 μm) sediment content in several ecosystems. Stocks also varied by dominant plant species within individual ecosystem types. At larger scales, marsh stocks were lowest in the Sonoran Desert region of Mexico, and swamp stocks differed among climate zones; otherwise stocks showed little correlation with ecoregion or latitude. More variability in SOC occurred among ecosystem types, and at smaller spatial scales (such as individual estuaries), than across regional climate gradients. These patterns can inform coastal conservation and restoration priorities across scales where preserving stored carbon and enhancing sequestration helps avert greenhouse gas emissions and maintains other vital ecosystem services. , Plain Language Summary Coastal blue carbon ecosystems such as seagrass meadows, marshes, mangroves, and other tidal wetlands, efficiently accumulate and store organic carbon in their sediments. For the west coast of North America, we investigated whether the amount of carbon stored in the sediment (“carbon stock”) differed by ecosystem type and whether differences were linked to local‐scale factors such as elevation and plant type or to regional‐scale factors such as latitude and climate conditions. We found the highest sediment carbon stocks in mangroves in Mexico and tidal swamps in the Pacific Northwest, ecosystems both dominated by woody plants. Tideflats had stocks at least as large as seagrass meadows, while marshes were intermediate among all ecosystems. Overall, differences in stocks were more related to factors like site elevation or the specific wetland sampled than broad geographic‐scale differences in climate conditions. Knowing how soil carbon stocks differ across various coastal conditions will help incorporate blue carbon information into planning and decision‐making in coastal conservation and restoration efforts. , Key Points Sediment organic carbon stocks were greatest in woody tidal wetlands, followed by marshes, mudflats, and seagrass habitats Unvegetated tideflats store much greater sediment carbon stocks than previously recognized Most stocks variability was explained by local‐scale drivers rather than large‐scale climate gradients