INTEREST in nitrous oxide (N2O) has increased considerably in the light of its deleterious effect on the ozone layer1, and contribution to the greenhouse effect2. There are many sources of atmospheric N2O, both anthropogenic (for example, combustion) and natural, but the global budget is still inadequately defined. Despite the fact that most of the world's oceans are close to equilibrium with the atmosphere3, water bodies depleted in oxygen have been identified as areas of N2O production4,5, and so the oceans represent a potential source of atmospheric N2O. Here we report elevated N2O concentrations in the northwest Indian Ocean, an area that exhibits upwelling and high oxygen consumption in the water column. We found that N2O was supersaturated in both oxygen-saturated surface waters (up to 246% N2O saturation) and oxygen-depleted sub-surface waters (1,264% N2O saturation). The calculated flux to the atmosphere indicated that upwelling in the northwest Indian Ocean (15-25° N) represents one of the most significant marine sources of N2O, contributing between 5 and 18% of the total marine flux from a surface area of only 0.43% of the world ocean. These data suggest that the oceanic flux of N2O to the atmosphere shows strong spatial heterogeneity which should be considered in global budgets and ocean-atmosphere models.