Background: Restrictive, very low-energy diets focused on rapid weight loss have proven to be effective in improving asthma outcome in obese patients, but their use in children and pubescents is controversial due its potential consequences in growth. More conservative, normocaloric schemes are suggested as a more suitable dietary approach for these patients. Methods: A randomized clinical trial was run of 51 pubertal adolescents with asthma and obesity, who were allocated to either an interventional 28-week program of normocaloric diet based on normal requirements for height and meal planning (n = 26) or a non-interventional (free diet) control group (n = 25). Asthma-related quality of life (AR-QOL, assessed by the Standardized Pediatric Asthma Quality of Life Questionnaire, PAQLQ[S]) and clinical indicators of asthma control were measured before and after the intervention period. Results: Diet intervention was associated with a significant improvement in AR-QOL in relation to baseline (Δ PAQLQ[S] scores) compared with controls, both in overall score (p < 0.001) and its subdomains (activity limitation, p < 0.001; symptoms, p < 0.002; emotional function, p < 0.001). The group with normocaloric diet observed a significant decrease in body mass index z-score, which correlated positively with the improvement in AR-QOL (Spearman's r = 0.51, p < 0.01), in addition to have significantly fewer events of acute attacks of asthma and nighttime awakenings, plus a non-significant reduction in the use of inhaled corticosteroids. No significant changes were observed in the pulmonary function tests. Conclusion: The normocaloric dietary intervention was associated with improvement of AR-QOL and some aspects of asthma control. Such structured dietary programs could probably have a role as a complementary non-pharmacological therapeutic strategy in obese pubertal adolescents with asthma.