This Randomized Controlled Trial (RCT) aims to evaluate the effectiveness of a brief Acceptance and Commitment Therapy (ACT)-based-intervention combined with treatment as usual (TAU) compared to TAU only in improving psychological conditions in a sample of adolescents with obesity (body mass index, BMI \> 97th centile for age and sex). Fifty consecutive adolescents (12-17 years) of both genders with obesity will be recruited among the patients hospitalized in a clinical center for obesity rehabilitation and randomly allocated into two experimental conditions: ACT + TAU vs TAU only. Both groups will attend a three-week in-hospital multidisciplinary rehabilitation program for weight loss. The ACT + TAU condition comprises a psychological intervention based on ACT combined with a standard psychological assessment and support to the hospitalization. The TAU comprises the standard psychological assessment and support to the hospitalization. At pre- to post-psychological intervention participants will complete the Avoidance and Fusion Questionnaire for Youth, the Psychological Well-Being Scale, the Depression Anxiety Stress Scale, the Difficulties in Emotion Regulation Scale, and the Emotional Eating subscale of the Dutch Eating Behavior Questionnaire. Repeated-measures ANOVAs (2x2) will be conducted. The study will assess the effectiveness of a brief ACT-based intervention for adolescents with obesity.