Introduction: Considering the incrediblly high incidence of obesity in adolescents, in this study, we have examined the association of DASH diet with 3-year body mass index change and risk of obesity in Tehranian adolescents. Materials and Method: This population based cohort study included 629 adolescents (aged 10 to 18 years) from the Tehran Lipid and Glucose Study (TLGS). Dietary data were collected using a valid and reliable food-frequency questionnaire. Anthropometric measures were assessed both at baseline and end of follow up. In adolescents, obesity was defined as body mass index ≥ 95th percentile for age and sex according to national reference curves. Results: Mean±SD for age and DASH score at baseline were 14.1±2.5 years and 24.1±4.2, respectively. Also, the mean±SD of body mass index gain and risk of incident obesity was 1.7±2.6 kg/m2 and 12% after 3 years follow up, respectively. After adjustment for potential confounding variables, higher DASH diet score and lower dietary intakes of sweet beverage were inversely associated with 3-year changes in body mass index (P<0.05). The odds ratio (95%CI) of developing obesity in the highest, compared with the lowest, quintile of DASH score was 0.2 (0.1- 0.7) (P for trend=0.025). Also, the lower intake of sweet beverage was significantly associated with reducing the risk of incident obesity (OR: 0.2; 95%CI: 0.1–0.6) (P for trend=0.003). Conclusion: A DASH- style dietary pattern is associated inversely with 3-year body mass index changes and risk of incident obesity in adolescents.