Marginal seas lie at the critical interface among terrestrial, oceanic, atmospheric, and anthropogenic systems and are important components of the global ocean carbon cycle. However, differences in observational methods, export depths, and regional environmental conditions have limited systematic assessments of particulate organic carbon (POC) export and biological carbon pump efficiency. Here, we synthesized 115 POC export observations from 41 marginal seas and combined them with in situ and satellite-derived primary productivity data to evaluate the magnitude, spatial patterns, and controls of upper-ocean POC export. The mean POC export flux was 7.41 ± 1.24 mmol C m⁻² d⁻¹, equivalent to an annual export of approximately 1.18 ± 0.20 Pg C yr⁻¹ and 14%–24% of global ocean POC export. Strong regional heterogeneity was observed: western boundary current regions showed high area-specific export fluxes; subpolar marginal seas exhibited both high regional export and biological pump efficiency; eastern boundary upwelling systems generally had moderate to high export; and Indian Ocean margins and some semi-enclosed seas showed lower values. Primary productivity and POC export displayed different spatial patterns, indicating that high production does not necessarily translate into high export. Carbon transfer through particle formation, aggregation, sinking, and remineralization is therefore critical to the effective strength of the biological carbon pump. Analyses of the South China Sea, Barents Sea, and other representative regions further showed that POC export is jointly regulated by hydrographic dynamics, nutrient supply, ecosystem structure, shelf processes, and thermal conditions. Sea surface temperature was not significantly related to POC export across global marginal seas, but growing-season sea surface temperature was positively correlated with POC export within Arctic marginal seas, suggesting greater sensitivity of high-latitude biological carbon pumps to ocean warming, sea-ice retreat, and changes in the productive season. These findings constrain the magnitude and regional variability of POC export in marginal seas and provide an integrated basis for improving global ocean carbon budgets and evaluating the role of marginal seas in the climate system.