Mangroves are highly productive coastal ecosystems that serve as critical reservoirs of blue carbon, providing essential ecological services. However, global mangrove coverage has declined substantially due to anthropogenic pressures and climate change. While restoration efforts are underway, accurately assessing their carbon sequestration potential remains a pivotal challenge in blue carbon research. Traditionally, mangrove carbon sinks have been evaluated based on stocks within biomass and sediments; yet, carbon fluxes driven by submarine groundwater discharge (SGD) can significantly reshape the overall carbon budget. This study quantifies SGD rates and associated carbon fluxes in a lagoon system containing both natural and artificially restored mangroves. Utilizing a three-end-member mixing model and a radon isotope mass balance approach, we found that the SGD rate in artificial mangroves was only 30% of that observed in natural stands. Despite this disparity in water flux, the SGD-derived fluxes of dissolved inorganic carbon (DIC) and dissolved organic carbon (DOC) in artificial mangroves were comparable to those in natural ecosystems. Notably, this lateral export of DIC and DOC represents a significant component of the mangrove carbon sink. In contrast, SGD in artificial mangroves transported methane (CH₄) to surface waters significantly higher than in natural mangroves. This elevated CH₄ export suggests enhanced outgassing to the atmosphere, which may partially offset the net carbon sink capacity of restored areas. These findings underscore that SGD processes and greenhouse gas dynamics must be integrated into blue carbon accounting frameworks to ensure accurate assessments of restoration efficacy.