Submarine groundwater discharge is defined as any flow of water at continental margins from the seabed to the coastal ocean, regardless of fluid composition or driving force. The flux of submarine groundwater discharge has been hypothesized to be a pathway for enriching coastal waters in nutrients, carbon and metals. Here, we estimate the submarine groundwater flux from the inventory of 228Ra in the upper Atlantic Ocean, obtained by interpolating measurements at over 150 stations. Only 46% of the loss in 228Ra from radioactive decay is replenished by input from dust, rivers and coastal sediments. We infer that the remainder must come from submarine groundwater discharge. Using estimates of 228Ra concentrations in submarine groundwater discharge, we arrive at a total flux from submarine groundwater discharge of 2-4×1013 m 3yr1, between 80 and 160 of the amount of freshwater entering the Atlantic Ocean from rivers. Submarine groundwater discharge is not a freshwater flux, but a flux of terrestrial and sea water that has penetrated permeable coastal sediments. Our assessment of the volume of submarine groundwater discharge confirms that this flux represents an important vehicle for the delivery of nutrients, carbon and metal to the ocean. © 2008 Nature Publishing Group.