Background: Research regarding the interaction of ambient air pollution and overweight on prehypertension is scarce. We aimed to test whether overweight modifies the association between long-term exposure to ambient air pollution and prehypertension in Chinese adults. Methods: A total of 16,188 Chinese adults, aged 18-74 years old, from 33 communities in 3 Northeastern Chinese cities were evaluated. Three-year average levels of particles with an aerodynamic diameter ≤ 10 μm (PM10), sulfur dioxides (SO2), nitrogen dioxides (NO2), and ozone (O3) were calculated at monitoring stations. Generalized additive models and 2-level regression analyses were applied. Results: We observed significant interactions between air pollutants and overweight on prehypertension and blood pressure. The associations of PM10, SO2, NO2, and O3 with prehypertension were significant among overweight participants (Prevalence Rate Ratios (PRRs) per interquartile range (IQR) of air pollutants: 1.14-1.20), but not among normal weight participants (PRRs: 0.98-1.04). PM10, SO2, and O3 were significantly associated with systolic blood pressure (SBP), and the magnitudes of these associations were higher among overweight adults (increases in SBP per IQR of air pollutants: 1.82-4.53 mmHg) than those among normal weight adults (increases in SBP: 0.42-0.61 mmHg). For diastolic blood pressure (DBP), significant associations were mainly observed in overweight participants (increases in DBP: 0.80-1.63 mmHg). Further stratified analyses showed that all these interactions were stronger in women, the older, and participants living in areas with lower income levels or higher population density. Conclusions: Being overweight may enhance the effects of ambient air pollution on prehypertension and blood pressure in Chinese adults.