Although long-term exposure to fine particulate matter (particles ≤ 2.5 μm in diameter [PM2.5]) has been linked to adverse cardiovascular outcomes, evidence remains limited regarding the effects of PM2.5 at concentrations below current regulatory standards on hypertension-related hospitalization. In this study, we examined the association between long-term exposure to PM2.5 concentrations below the U.S. Environmental Protection Agency annual standard of 9 μg/m3 and the risk of hypertension-related hospitalization among Medicare beneficiaries aged ≥65 years across the contiguous United States from 2017 to 2022 using a causal inference approach. We constructed a national cohort of more than 26 million older adults from the Medicare database residing in ZIP codes where annual PM2.5 concentrations consistently remained below 9 μg/m3 throughout the study period. To address unmeasured confounding, we used a double-negative control approach. Secondary analyses compared the main findings with results from conventional quasi-Poisson regression models and examined effect modification by population subgroups. Each 1-μg/m3 increase in the annual PM2.5 concentration was associated with a 2.8% (95% CI: 2.5-3.2) increase in hypertension-related hospitalization among all beneficiaries, comparable with estimates from conventional quasi-Poisson regression models. Greater vulnerability was observed among women, residents of the Midwest and Northeast, those living in rural or suburban areas, and individuals in more socioeconomically deprived neighborhoods. These findings suggest that even PM2.5 exposures below current regulatory thresholds in the United States may contribute to increased hypertension risk, emphasizing the need to reconsider existing air quality standards to better protect public health.