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This study aims to quantify soil organic C density (SOCD) in tropical semiarid mangroves and to compare these results to SOCD under the main tropical semiarid vegetation units (VU), aiming to better understand the role of semiarid mangroves in C sequestration, as well as to provide a basis for a better quantification of human impacts on this ecosystem. SOCD was quantified in two sets of calculations: at a 40-cm soil depth and considering the same soil mass (equivalent SOC). Mangroves can be considered the main organic C sink in the tropical semiarid environment, storing twice as much organic C per area as semiarid VUs, when considering the upper 40-cm soil layer and almost 6 times as much organic C per area when considering the same soil mass. Anthropogenic impacts (disposal of nutrients and vegetation removal) on the studied mangroves decreased SOCD and resulted in a 4.94 +/- 0.74Tg CO2 emission, equivalent to 5.2% of the Brazilian annual CO2 emission by fossil fuels. Compared to mangroves from tropical humid regions, SOCD in tropical semiarid mangroves is considerably lower.