Telomere length (TL) is a critical biomarker of cellular aging and has been increasingly associated with fine particulate matter (PM2.5) exposure. This study investigates the profile of leukocyte TL with ambient PM_2.5 exposure among adults residing in Chiang Mai, Thailand. A total of 182 participants were enrolled, consisting of 72% females, aged between 23 and 66 years, and stratified into three age groups: under 35 (n = 36), 35–49 (n = 88), and 50 years old or older (n = 58). Relative TL was measured from blood leukocytes using quantitative polymerase chain reaction (qPCR), with results expressed as the telomere-to-single-copy gene (T/S) ratio. PM_2.5 levels were retrieved from local monitoring stations and averaged across lag periods (in weeks). The mean TLs for participants aged <35, 35–49, and ≥50 were 0.83 ± 0.17, 0.81 ± 0.15, and 0.82 ± 0.13, respectively (P > 0.05), with no significant differences observed in age, BMI, smoking status, or gender among the groups. The mean PM2.5 concentration for Lag03 (3 weeks prior) was 45.9 ± 2.92 µg/m³, ranging from 42.6 to 51.2 µg/m³. Adjusting for age, BMI, smoking status, and gender, the multivariable linear regression model showed that PM_2.5 Lag03 was a significant predictor of TL, indicating that higher short-term PM_2.5 exposure was associated with longer relative TL (B = 0.019, 95% CI = 0.11–0.29, β = 0.368, p < 0.001). In this study, TL was similar across the three age groups, while the overall TL values were much shorter than those found in a previous study of healthy nonsmokers in Chiang Mai (TL range: 1.35–1.00). Differences may stem from environmental exposures, measurement methods, or lifestyle. Further studies should explore the effects of PM2.5-bound chemicals, such as polycyclic aromatic hydrocarbons (PAHs) and heavy metals, especially in the context of long-term exposure.