Objective: This study prospectively assessed the association of the inflammatory potential of a diet using the dietary inflammatory index (DII) with average yearly weight changes and incident overweight/obesity. Methods: Seven thousand and twenty-seven university graduates with body mass index <25 from the Seguimiento Universidad de Navarra (SUN) cohort were followed up during a median of 8.1 years. The DII, a validated tool based on scientific evidence to appraise the relationship between dietary parameters and inflammatory biomarkers, was used. A validated food-frequency questionnaire was used to assess intake of total energy, food, and nutrients, from which DII scores were calculated at baseline and after 10 years of follow-up. Results: After a median follow-up of 8.1 years, 1,433 incident cases of overweight or obesity were observed. Hazard ratios for overweight/obesity were calculated, including multivariable time-dependent Cox regression models with repeated measures of diet. The hazard ratio for subjects in the highest quartile (most pro-inflammatory diet) was 1.32 (95% confidence interval 1.08-1.60) compared with participants in the lowest quartile (most anti-inflammatory diet), with a significant linear dose-response relationship (P=0.004). Consistently, increases in average yearly weight gains were significantly associated with proinflammatory diets. Conclusions: A proinflammatory diet was significantly associated with a higher annual weight gain and higher risk of developing new-onset overweight or obesity.