Aim: This research aimed to explore the impact of school-based Lego Therapy groups for adolescents with a diagnosis of ASD in mainstream schools. Method/Rationale: School staff were trained in the delivery of the social skills intervention to groups comprising one adolescent with ASD and two typically developing peers. A non-concurrent multiple baseline across participants design was employed to examine the impact of the intervention for six adolescent males with ASD. Findings: Visual analysis, PAND effect sizes and Tau-U statistical analyses demonstrated the large positive impact of the intervention on duration of social engagement and frequency of social initiations, responses and positive social behaviours for five out of six participants. The final participant withdrew from the research. Parents and teachers saw some evidence of generalisation of skill to home and other aspects of school life but this was not consistent for all participants. Fidelity of implementation was maintained, suggesting the approach is appropriate for delivery in school settings by school staff. Conclusion: The results of this study suggest that Lego Therapy groups can be an effective school-based social skills intervention for adolescents with ASD. Limitations: This research was limited by the small sample size. Future research should extend the evidence-base of Lego-based social skill groups, examining the impact of the intervention for a wider range of students with differing needs.