Background: We investigated the joint associations of habitual physical activity (PA) and long-term exposure to fine particulate matter (PM _2.5 ) with the development of hypertension in a longitudinal cohort in Taiwan. Methods: We selected 140 072 adults (≥18 years of age) without hypertension who joined a standard medical screening program with 360 905 medical examinations between 2001 and 2016. PM _2.5 exposure was estimated at each participant’s address using a satellite data-based spatiotemporal model with 1 km ² resolution. Information on habitual PA and a wide range of covariates was collected using a standard self-administered questionnaire. We used the Cox regression model with time-dependent covariates to examine the joint associations. Results: The mean age of all observations was 41.7 years, and 48.8% were male. The mean value for systolic and diastolic blood pressure was 112.5 and 68.7mm Hg, respectively. Approximately 34.2% of all observations were inactive (0 metabolic equivalence values-hours), 29.8% had moderate-PA (median [interquartile range]; 3.75 [3.38 to 4.38] metabolic equivalence values-hours), and 36.0% had high-PA (15.7 [10.3 to 24.8] metabolic equivalence values-hours). The mean±SD of PM _2.5 was 26.1±7.3 μg/m ³ . The prevalence of cardiovascular disease, diabetes mellitus, and cancer was 2.1%, 2.9%, and 1.5%, respectively. After adjusting for a wide range of covariates (including a mutual adjustment for PA or PM _2.5 ), a higher PA level was associated with a lower risk of hypertension (hazard ratio [HR] for the moderate- and high-PA was 0.93 [95% CI, 0.89–0.97] and 0.92 [95% CI, 0.88–0.96], respectively, as compared with the inactive-PA), whereas a higher level of PM _2.5 was associated with a higher risk of hypertension (HR for the moderate- and high-PM _2.5 was 1.37 [95% CI, 1.32–1.43] and 1.92 [95% CI, 1.81–2.04], respectively, as compared with the low-PM _2.5 group]. No significant interaction was observed between PA and PM _2.5 (HR 1.01 [95% CI, 1.00–1.02]). Conclusions: A high-PA and low PM _2.5 exposure were associated with a lower risk of hypertension. The negative association between PA and hypertension remained stable in people exposed to various levels of PM _2.5 , and the positive association between PM _2.5 and hypertension was not modified by PA. Our results indicated that PA is a suitable hypertension prevention strategy for people residing in relatively polluted regions.