Interventions aiming to reduce social inequalities of weight status in adolescents usually focus on lifestyle behaviours, but their effectiveness is limited. This study analysed the effect of achieving levels of dietary intake (DI) and/or physical activity (PA) guidelines on reducing social inequalities in weight status among adolescents. We included adolescents from the PRomotion de l’ALIMentation et de l’Activité Physique – INÉgalité de Santé (PRALIMAP-INÈS) trial with weight status data available at baseline and 1-year follow-up (n=1130). PA and DI were measured using the International Physical Activity Questionnaire and a validated food frequency questionnaire, respectively. We estimated the likelihood of a 1-year reduction in body mass index z-score (BMIz) and population risk difference (PRD) under hypothetical DI and PA levels and socioeconomic status using the parametric G-formula. When advantaged and less advantaged adolescents maintained their baseline DI and PA, we found social inequalities in weight status, with a PRD of a 1-year reduction in BMIz of -1.6% (-3.0%; -0.5%). These inequalities were not observed when less advantaged adolescents increased their proportion of achieving DI guidelines by 30% (PRD=2.2% [-0.5%; 5.0%]) unlike the same increase in PA (PRD= -3.9% [-6.8%; -1.3%]). Finally, social inequalities of weight status were not observed when levels of achievement of both PA and DI guidelines increased by 30% (PRD= 2.2% [-0.5%; 4.0%]). Enhancing DI rather than PA could be effective in reducing social inequalities in weight status among adolescents. Future interventions aiming to reduce these inequalities should mostly target DI to be effective.