The placenta plays a crucial role in fetal growth and development through adaptive responses to perturbations of the maternal environment. We investigated the association between placental 3-nitrotyrosine (3-NTp), a biomarker of oxidative stress, and exposure to air pollutants during various time windows of pregnancy. We measured the placental 3-NTp levels of 330 mother-newborn pairs enrolled in the Environmental Influence on Ageing in Early Life (ENVIRONAGE) Study, a Belgian birth cohort study (2010–2013). Daily concentrations of particulate matter with an aerodynamic diameter ≤2.5 µm (PM2.5), black carbon (BC), and nitrogen dioxide were interpolated for each mother's residence using a spatiotemporal interpolation method. Placental 3-NTp levels, adjusted for covariates, increased by 35.0% (95% confidence interval (CI): 13.9, 60.0) for each interquartile-range increment in entire-pregnancy PM2.5 exposure. The corresponding estimate for BC exposure was 13.9% (95% CI: −0.21, 29.9). These results were driven by the first (PM2.5: 29.0% (95% CI: 4.9, 58.6); BC: 23.6% (95% CI: 4.4, 46.4)) and second (PM2.5: 39.3% (95% CI: 12.3, 72.7)) gestational exposure windows. This link between placental nitrosative stress and exposure to fine particle air pollution during gestation is in line with experimental evidence on cigarette smoke and diesel exhaust exposure. Further research is needed to elucidate potential health consequences experienced later in life through particle-mediated nitrosative stress incurred during fetal life.