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Exposure to ambient air pollution has been associated with greater risk of elevated blood pressure (BP) in adults and children. Recent evidence suggests that air pollution exposure in pregnancy may also portend increased risk for the next generation; however, few studies have examined this relationship. We conducted a prospective study of 1293 mothers in the Boston Birth Cohort (enrolled 1998–2012) and their children who had follow-up visits between 3 and 9 years of age and complete exposure and outcome data. Our primary exposure, ambient particulate matter ≤2.5 µm (PM2.5) concentration during pregnancy, was estimated by matching mother’s residential address to the US Environmental Protection Agency’s air quality monitors. We defined our primary outcome child systolic BP (SBP) percentile according to US reference (Fourth Report) and classified elevated BP as SBP ≥90th percentile. Our multivariable-adjusted cubic spline showed a sharp increase in offspring SBP percentile and risk for elevated BP when third-trimester PM2.5 concentration was ≥13 μg/m3. The highest versus lowest tertile of third-trimester PM2.5 exposure was associated with a 4.85 (95% confidence interval: 1.38–8.37) percentile increase in child SBP or a 1.61 (95% confidence interval: 1.13–2.30) times higher risk of child elevated BP. A 5-μg/m3 increment in PM2.5 during the third trimester was associated with a 3.49 (95% confidence interval: 0.71–6.26) percentile increase in child SBP or a 1.47 (95% confidence interval: 1.17–1.85) times higher risk of elevated BP. Our findings suggest that exposure to ambient PM2.5 during the third trimester of pregnancy is associated with elevated BP in children, ages 3 to 9 years.