Objective: Although the adoption of air pollution mitigation strategies has greatly improved air quality over recent decades, their beneficial effects on health remain underexplored. We therefore investigated the association between reductions in PM_2.5 exposure and hypertension risk. Methods: This prospective cohort study utilized data from the Taiwan MJ Cohort (2005–2016) and the UK Biobank (UKB, 2006–2015), representing medium- and low-PM_2.5 areas, respectively. Ground-level PM_2.5 data were derived from the Atmospheric Composition Analysis Group and matched to each participant’s residential address. Participants whose baseline exposure exceeded the cohort-specific median PM_2.5 level were included (MJ Cohort: n = 38,397; UKB: n = 66,837). Covariates and outcome definitions were standardized across cohorts, and Cox regression models with inverse probability weighting (IPW) were used to analyze the associations between PM_2.5 reduction and the 5-year risk of hypertension. A parallel analysis was performed among individuals with pre-hypertension. Results: In the MJ Cohort, participants who experienced a reduction in PM_2.5 exposure had a significantly lower risk of developing hypertension compared with those who continuously lived in high-PM_2.5 areas (hazard ratio [HR] = 0.83, 95% confidence interval [CI]: 0.73–0.95). Each 1 µg/m³ decrease in PM_2.5 exposure was also associated with a reduced risk of hypertension (HR = 0.97, 95% CI: 0.95–0.99). In the UKB, a significant association was observed only when PM_2.5 exposure was analyzed as a continuous variable (per 1 µg/m³ decrease: HR = 0.85; 95% CI: 0.76–0.94). Notably, the benefits of reduced PM_2.5 exposure were even more pronounced among individuals with pre-hypertension in both cohorts. Conclusion: Analysis of two large cohorts from regions with varying air pollution levels demonstrates that reductions in long-term PM_2.5 exposure are consistently associated with a lower risk of developing hypertension, providing direct evidence of the substantial health benefits of improved ambient PM_2.5 levels.