The oxygen isotopic composition (Δ<sup>17</sup>O) of atmospheric nitrate is a function of the relative abundance of atmospheric oxidants (O<sub>3</sub>, RO<sub>x</sub>=OH+HO<sub>2</sub>+RO<sub>2</sub>) and the formation pathway of nitrate from its precursor NO<sub>x</sub> (=NO+NO<sub>2</sub>). Coupled observations and modeling of nitrate Δ<sup>17</sup>O can be used to quantify the relative importance of chemical formation pathways leading to nitrate formation and reduce uncertainties in the budget of reactive nitrogen chemistry in the atmosphere. We present the first global model of atmospheric nitrate Δ<sup>17</sup>O and compare with available observations. The largest uncertainty for calculations of nitrate Δ<sup>17</sup>O is the unconstrained variability in the Δ<sup>17</sup>O value of tropospheric ozone. The model shows the best agreement with a global compilation of observations when assuming a Δ<sup>17</sup>O value of tropospheric ozone equal to 35‰ and preferential oxidation of NO<sub>x</sub> by the terminal oxygen atoms of ozone. Calculated values of annual-mean nitrate Δ<sup>17</sup>O in the lowest model layer (0–200 m above the surface) vary from 7‰ in the tropics to 41‰ in the polar-regions. The global, annual-mean tropospheric inorganic nitrate burden is dominated by nitrate formation via NO<sub>2</sub>+OH (76%), followed by N<sub>2</sub>O<sub>5</sub> hydrolysis (18%) and NO<sub>3</sub>+DMS/HC (4%). Calculated nitrate Δ<sup>17</sup>O is sensitive to the relative importance of each nitrate formation pathway, suggesting that observations of nitrate Δ<sup>17</sup>O can be used to quantify the importance of individual reactions (e.g. N<sub>2</sub>O<sub>5</sub> hydrolysis) leading to nitrate formation if the Δ<sup>17</sup>O value of ozone is known.