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Existing analyses of salt marsh carbon budgets rarely quantify carbon loss as CO₂through the air–water interface in inundated marshes. This study estimates the variability of partial pressure of CO₂(pCO₂) and air–water CO₂fluxes over summer and fall of 2014 and 2015 using high‐frequency measurements of tidal waterpCO₂in a salt marsh of the U.S. northeast region. Monthly mean CO₂effluxes varied in the range of 5.4–25.6 mmol m⁻²marsh d⁻¹(monthly median: 4.8–24.7 mmol m⁻²marsh d⁻¹) during July to November from the tidal creek and tidally‐inundated vegetated platform. The source of CO₂effluxes was partitioned between the marsh and estuary using a mixing model. The monthly mean marsh‐contributed CO₂effluxes accounted for a dominant portion (69%) of total CO₂effluxes in the inundated marsh, which was 3–23% (mean 13%) of the corresponding lateral flux rate of dissolved inorganic carbon (DIC) from marsh to estuary. Photosynthesis in tidal water substantially reduced the CO₂evasion, accounting for 1–86% (mean 31%) of potential CO₂evasion and 2–26% (mean 11%) of corresponding lateral transport DIC fluxes, indicating the important role of photosynthesis in controlling the air–water CO₂evasion in the inundated salt marsh. This study demonstrates that CO₂evasion from inundated salt marshes is a significant loss term for carbon that is fixed within marshes.