Macroalgal forests have been increasingly recognized as important contributors to marine carbon dioxide removal because of their high productivity and extensive global distribution. However, their carbon sequestration potential remains poorly constrained, as conventional estimates rely primarily on the export and long-term burial of organic carbon, processes that are difficult to quantify directly. Here, we quantified the net carbon sequestration of a temperate marine forest on the east coast of Korea using a carbon–budget approach that integrates biological dissolved inorganic carbon (DIC) drawdown, air–sea CO₂ exchange, and lateral DIC transport based on multi-year field observations. To isolate the carbon uptake attributable to the marine forest ecosystem, an equivalent carbon budget was independently estimated for adjacent offshore waters and subtracted from the marine forest budget. Biological DIC drawdown represented the largest component of carbon uptake, while lateral advection substantially modified the net carbon budget by exporting biologically altered seawater to offshore regions. The resulting net marine forest carbon sequestration was estimated to be 3.3 ± 0.2 t CO₂ ha^-1 yr^-1, indicating that the ecosystem removed substantially more carbon than the surrounding coastal ocean. These results demonstrate that integrating biological and physical carbon pathways provides a more robust assessment of macroalgal carbon sequestration than conventional export-based approaches and offers a practical framework for future blue carbon accounting and marine carbon dioxide removal assessments.