Résumé : Purpose: Overweight and obesity are associated with poor mental health in adolescents. However, little is known about whether the influence of overweight and obesity on mental well-being is mediated by self-perceived body weight. Exploring the mechanisms underlying the relationships between obesity and mental well-being is of interest to policy makers and others working in the field of adolescent health. Methods: This study was based on nationally representative data from adolescents (age 15 years) who participated in the 2017/2018 Health Behaviour in School-aged Children study (47 countries, N = 76,998). Mixed regression models that included gender and socioeconomic status as covariates were used to identify associations between weight status and mental well-being (life satisfaction and subjective health complaints) and to explore whether self-perceived body weight (feeling too thin or too fat) has a mediating effect. Associations between weight status, self-perceived weight, and mental well-being were further assessed country by country. Results: Self-perceived body weight mediated the observed associations between overweight or obesity and mental well-being. Perceiving one's body weight as “too thin” or “too fat” was associated with poorer mental well-being, regardless of weight status. Self-perceived body weight varied by gender, socioeconomic status, and country. Discussion: Self-perceived body weight may explain, to a greater extent than body mass index, variation in mental well-being among adolescents. These results are important to policy makers, clinicians, and others targeting adolescent health.