Background/Objectives: The rising prevalence of pediatric obesity highlights the urgent need for effective lifestyle interventions that improve diet quality, in line with global health objectives. Tackling obesity through planetarian dietary practices not only enhances individual health but also mitigates the environmental impact of food systems. The EAT-Lancet Commission’s plant-based dietary recommendations underscore the dual benefit of promoting human health while supporting environmental sustainability. This study aims to assess changes in adherence to a planetarian diet, measured through planetary environmental impact indices, following a lifestyle intervention in a pediatric population. Methods: In this randomized controlled trial, 107 participants with abdominal obesity were assigned to either a usual care group or an intensive intervention group, the latter following a moderately hypocaloric Mediterranean diet combined with nutritional education. Adherence to the EAT-Lancet diet was evaluated using both the EAT-Lancet Diet Score and the EAT-Lancet Diet Index. Results: The intensive lifestyle intervention significantly improved adherence to the EAT-Lancet diet, leading to notable reductions in body mass index (BMI), weight, and waist circumference, alongside improvements in both anthropometric and clinical outcomes. Conclusions: This study demonstrates that intensive lifestyle interventions in children and adolescents with abdominal obesity can reduce BMI-SDS (BMI-standard deviation score) and improve adherence to planetarian dietary patterns, leading to enhanced health outcomes. Further research is needed to evaluate the long-term effects of such interventions and to determine their broader applicability across diverse pediatric populations.