Elevated blood pressure (BP) has been proposed as a possible pathophysiological mechanism linking exposure to ambient air pollution and the increased risk of cardiovascular mortality and morbidity. In this study, we investigated the hourly relationship between ambient air pollutants and BP. BP measurements were extracted from the electronic health record database of the Seoul National University Bundang Hospital from February 2015 to June 2017. A total of 98,577 individual BP measurements were matched to the hourly levels of air pollutants. A generalized additive model was constructed for hour lags of 0-8 of air pollutants adjusting for age, sex, meteorological variables, and time trend. Systolic BP was shown to be significantly lower at 2-4 hours and 3-5 hours after increased levels of SO2 and CO, respectively (0.24 mmHg and 0.26 mmHg for an interquartile range, respectively). In contrast, O3 and NO2 were associated with significantly increased systolic BP at 3-5 lag hours and at 0-2 lag hours, respectively. BP elevation in association with O3 and NO2 was shown to be significantly greater in hypertensive patients than normotensive subjects. Our findings suggest that short-term exposure to air pollution may be associated with elevated BP.