As a main source of nitrate for the pycnocline in the global ocean, the ocean south of 30 degrees S plays a pivotal role in the global marine biogeochemical cycle. Nitrate in the pycnocline often shows a more stable relationship with density than with depth. Hence, it is important to understand the nitrate structure and its relationship with density in the ocean south of 30 degrees S. In the present study, the nitrate-density relationship and the mechanisms for its variability are studied based on the data from more than 60,000 profiles. Through analysis, we find that nitrate is approximately linearly correlated with potential density in the nitracline, except for the western boundary current zones. The upper bounds of the nitracline depend on the mixed layer and euphotic layer depth. The lower bounds are related to the depth of maximum nitrate, which mainly vary meridionally with sharp changes near the Subantarctic Front. The diapycnal gradients of nitrate also show meridional variability, with large magnitudes corresponding to thick Subantarctic Mode Water, low diapycnal mixing in the nitracline, and high biological uptake in the euphotic layer. The nitrate-density relationship can be applied to predict nitrate concentrations based on the observed temperature and salinity profiles and support some detailed studies on the physical-biogeochemical interactions. Two examples show that the predicted nitrate data could help better resolve the effects of ocean circulations and mesoscale eddies on nitrate than the existing observed nitrate profiles.