Background: The outcomes of counselling in UK primary schools are still largely unknown, and no clear predictors of change have been identified. This study is the largest evaluation to date of change and predictors of change in this sector. Method: The outcomes of Place2Be’s play-based counselling intervention for 3222 distressed children, aged 4-12, in 178 schools across the UK were evaluated. A naturalistic quantitative design was used, and the primary outcome was changed from baseline to endpoint on the teacher-rated Strengths and Difficulties Questionnaire (SDQ) Total Difficulties score. Outcome predictors were analysed using multilevel modelling. Results: The intervention was associated with significant reduction in psychological distress, with an effect size (Cohen’s d) of .48 and clinical improvement of 50.3% of children. Greater improvements were found for children with higher baseline levels of distress, girls and bilingual children, while improvement of the children with a statement of educational needs was progressively compromised with increasing level of special educational needs. Discussion: Robust evidence now exists that a play-based counselling model is associated with significant reductions in psychological distress in primary school children. However, more qualitative and quantitative research is needed to assess the effectiveness of this intervention and the key factors that predict change.