Background <p>New evidence suggests that air pollution is associated with type 2 diabetes (T2D), but the underlying mechanisms are unclear. Therefore, we aimed to investigate the relationship between air pollution exposure and T2D and to quantify the role of sleep quality in this association.</p> Methods <p>A total of 226,188 participants free of T2D at baseline were enrolled in this prospective cohort study. Annual average concentrations of particulate matter (PM_2.5 and PM₁₀) and nitrogen oxides (NO₂, NO_X) were assessed using a land-use regression. Stratified Cox regression models and Aalen additive hazards models were used to estimate the association between air pollutants and T2D. Causal mediation modelling was used to estimate the indirect impact of air pollutants on T2D through sleep quality.</p> Results <p>The adjusted hazard ratio (HR) of T2D for a 10-µg/m³ increase in PM_2.5, PM₁₀, NO₂, and NO_X were 1.76 (95%CI: 1.37–2.27), 1.61 (95%CI: 1.36–1.91), 1.05 (95%CI:1.01–1.09), and 1.03 (95%CI:1.02–1.05), respectively. Additionally, for a 10-µg/m³ increase in PM_2.5, PM₁₀, NO₂ and NO_X, estimates of 161 (95%CI: 91–231), 124 (95%CI: 80–168), 14 (95%CI: 4–24) and 10 (95%CI: 5–15) additional T2D cases per 10⁵ persons per year, respectively, were detected. Sleep quality mediated 19.12% to 37.28% of the air pollution-T2D relationship.</p> Conclusions <p>The study revealed that long-term exposure to air pollution increased the risk of T2D incidence, with sleep quality serving as an important mediating factor. There are two modifiable risk factors for T2D identified and emphasized in our study: air pollution and sleep quality, which shed light on prospective primary prevention.</p> Graphical abstract <p></p>