This case report describes the nutritional management and long-term outcomes of an adolescent undergoing bariatric surgery. A 13-year-old female patient with morbid obesity complicated by nonalcoholic steatohepatitis (NASH), impaired glucose tolerance (IGT), and polycystic ovary syndrome (PCOS) underwent sleeve gastrectomy in July 2021. The patient achieved significant weight loss, with a total weight loss of 43.3% and a body mass index reduction of 16.6 kg/m2 at 3 years post-surgery, which further increased to 18.6 kg/m2 by the fourth year. Remission of NASH, IGT, and PCOS was observed after 1 year. Postoperatively, vitamin D deficiency developed, whereas other biochemical parameters remained within normal reference ranges. Adherence to recommended nutritional supplementation was suboptimal; however, with continuous nutritional education and regular follow-up, the patient ultimately established and maintained a balanced dietary pattern. The case highlights the effectiveness of bariatric surgery in achieving sustained weight loss and improving obesity-related comorbidities in adolescents, while underscoring the critical role of continuous nutritional management, patient education, and individualized multidisciplinary care in supporting long-term postoperative success.