Objectives: Perfectionism is considered to be an underlying mechanism of relevance to a broad array of indicators of psychological distress. The current research examined the impact of a three-session intervention targeting perfectionism in children on perfectionism, self-criticism, and well-being. Design: The design of the current study can be considered quasi-experimental as the intervention and control classes were not randomly allocated but decided by convenience factors at the school level. Methods: Students (aged 10.08–12.79 years) were allocated to the intervention (N = 107, 41 boys) or control condition (N = 105, 33 boys), completing self-report assessments on perfectionism, self-criticism, and well-being at baseline, post-intervention, and 3-month follow-up. Results: At post-intervention, children in the intervention group had significantly lower perfectionism than the control group (d = 0.35, 95% confidence intervals [CI]: 0.07–0.62) and at 3-month follow-up had significantly higher levels of well-being (d = 0.33, 95% CI: 0.06–0.60). As predicted by theory, decreases in perfectionism mediated the relationship between condition and improved well-being. Conclusions: This exploratory study provides evidence for the usefulness of a brief universal prevention programme targeting perfectionism. Future research should use more robust designs, explore longer-term effects, and the impact on a wider range of variables, including scholastic achievement. Practitioner points: Clinical implications. Perfectionism linked with negative outcomes in children can be decreased in a classroom setting. Decreasing perfectionism leads to improved well-being in children. Limitations. More rigorous designs along with better assessment of perfectionism are required in further evaluations. The impact of perfectionism on scholastic achievement in children requires further investigation.