The global agri-food system relies on synthetic nitrogen (N) fertilisation to increase crop yields, yet the use of synthetic N fertiliser is unsustainable. In this study we estimate global greenhouse (GHG) emissions due to synthetic N fertiliser manufacture, transportation, and field use in agricultural systems. By developing the largest field-level dataset available on N2O soil emissions we estimate national, regional and global N2O direct emission factors (EFs), while we retrieve from the literature the EFs for indirect N2O soil emissions, and for N fertiliser manufacturing and transportation. We find that the synthetic N fertiliser supply chain was responsible for estimated emissions of 1.25 GtCO2e in 2018, representing 21.5% of agriculture direct emissions and 2.4% of global GHG emissions. Synthetic N fertiliser production accounted for 35.2% of total synthetic N fertiliser-associated emissions, while field emissions accounted for 62.4% and transportation accounted for the remaining 2.4%.The top four emitters together, China, India, USA and EU28 accounted for 63% of the total. Historical trends reveal the great disparity in total and per capita N use and efficiency in regional food production. Reducing overall production and use of synthetic N fertilisers offers large mitigation potential and in many cases realisable potential to reduce emissions.