Purpose <p>To investigate whether children with obesity who decide not to participate in a lifestyle intervention experience a more negative long-term development in psychosocial well-being.</p> Methods <p>This observational study included 403 children aged 5–10&#xa0;years with obesity. Children who decided&#xa0;not&#xa0;to&#xa0;participate (<i>n</i> = 71) were compared to those not invited (not-invited group, <i>n</i> = 225) and those attending the lifestyle intervention (intervention group, <i>n</i> = 107). Psychosocial well-being was assessed with a mean follow-up of 24.5 ± 4.5&#xa0;months using the self-reported Danish National Well-being Questionnaire. Data from national registries and mandatory health examinations were included. Within-group changes were analyzed using McNemar’s test, and group differences with multivariable logistic regression.</p> Results <p>Adjusted analyses showed no significant differences in well-being changes between groups. However, children who decided-not-to-participate reported an overall decrease in psychosocial well-being with significantly increased school-related loneliness and reduced classroom concentration and helpfulness over time. Similarly, the not-invited group reported reduced school-related classroom concentration, helpfulness, and enjoyment but improved loneliness and bullying over time.</p> Conclusions <p>Deciding not to participate in a lifestyle intervention for childhood obesity was associated with a more negative development in psychosocial well-being over time; however, causality needs to be determined.</p> <p><i>Clinical trial registration</i>: ClinicalTrials.gov identifier: NCT06705231.</p> <p>Level of evidence: Level III, longitudinal cohort study.</p>