Purpose: To investigate the effects of a multidisciplinary childhood obesity treatment programme on subjective evaluations of psychosocial well-being and quality of life. Methods: This longitudinal observational study included 1291 children, adolescents and young adults, 6–22 years of age, with overweight or obesity. At entry and after 2–82 months of obesity treatment, the patients evaluated the following domains of psychosocial well-being on a visual analogue scale: quality of life, mood, appetite, bullying, motivation for weight loss and body image satisfaction. The degree of overweight was calculated using a body mass index (BMI) standard deviation score (SDS) at each visit. Results: At entry, the mean BMI SDS was 2.81 (range: 1.35–6.65, 95% confidence interval (95% CI): 2.44–3.18). After a median of 14 months of treatment, the median reduction in BMI SDS was 0.29 (95% CI: 0.26–0.31, p < 0.0001). Improvements were observed in the domains of quality of life, mood, appetite, bullying and body image satisfaction (p < 0.0001). Larger reductions in BMI SDS were associated with greater improvements in the domains of quality of life (p = 0.001), mood (p = 0.04) and body image satisfaction (p < 0.0001), independent of BMI SDS at entry. However, improvements in psychosocial well-being were also observed in those increasing their BMI SDS (n = 315). Conclusions: In a large group of children and youths, psychosocial well-being improved during a multidisciplinary childhood obesity treatment programme, irrespective of the degree of obesity at treatment entry. Greater reductions in BMI SDS were associated with greater improvements in psychosocial well-being, but even in the group increasing their BMI SDS improvements were observed.