Ocean iron fertilization is being considered as a strategy for mitigating the buildup of anthropogenic CO₂ in the atmosphere. Assessment of this strategy requires consideration of its unintended consequences, such as an enhancement of ocean N₂O emissions. This feedback could offset the radiative benefit from the atmospheric CO₂ reduction significantly, because N₂O is a much more powerful greenhouse gas than CO₂ itself. Our model results show that the magnitude of this offsetting effect is substantial, but is highly dependent on the location and duration of fertilization. We find the largest offsets (of the order of 100%) when fertilization is undertaken in the tropics, particularly when it is of limited duration and size. Smaller, but still substantial effects are found when fertilization is undertaken elsewhere and over longer periods. These results suggest that any assessment of ocean fertilization as a mitigating option is incomplete without consideration of the N₂O feedback.