The highly saline, oxygen-saturated waters of the Arabian Gulf (hereafter the Gulf) sink to intermediate depths (200-300 m) when they enter the Arabian Sea, ventilating the World's thickest oxygen minimum zone (OMZ). Here, we investigate the impacts of a warming of the Gulf consistent with climate change projections on the intensity of this OMZ. Using a series of eddy-resolving model simulations, we show that the warming of the Gulf waters increases their buoyancy and hence limits their contribution to the ventilation of intermediate depths. This leads to an intensification of the OMZ and an increase in denitrification that depletes subsurface nitrate and limits deoxygenation at depth. The projected future concomitant increase of Gulf salinity only partially reduces the OMZ intensification. Our findings highlight the importance of the Arabian marginal seas for the biogeochemistry of the North Indian Ocean and stress the need for improving their representation in global climate models.