Background Social skills interventions are commonly recommended to help children and young people with autism spectrum disorder develop social skills, but some struggle to engage in these interventions. LEGO^® (LEGO System A/S, Billund, Denmark) based therapy, a group social skills intervention, aims to be more interesting and engaging. Objective To evaluate the clinical effectiveness of LEGO^® based therapy on the social and emotional skills of children and young people with autism spectrum disorder in school settings compared with usual support. Secondary objectives included evaluations of cost-effectiveness, acceptability and treatment fidelity. Design A cluster randomised controlled trial randomly allocating participating schools to either LEGO^® based therapy and usual support or usual support only. Setting Mainstream schools in the north of England. Participants Children and young people (aged 7–15 years) with autism spectrum disorder, their parent/guardian, an associated teacher/teaching assistant and a facilitator teacher/teaching assistant (intervention schools only). Intervention Schools randomised to the intervention arm delivered 12 weekly sessions of LEGO^® based therapy, which promotes collaborative play and encourages social problem-solving in groups of three children and young people with a facilitator (trained teacher or teaching assistant). Participants received usual support from school and community services. Participants in the control arm received usual support only. Research assistants and statisticians were blind to treatment allocation. Main outcome measure The social skills subscale of the Social Skills Improvement System (SSIS), completed by the children and young people’s unblinded teacher pre randomisation and 20 weeks post randomisation. The SSIS social skills subscale measures social skills including social communication, co-operation, empathy, assertion, responsibility and self-control. Participants completed a number of other pre- and post-randomisation measures of emotional health, quality of life, loneliness, problem behaviours, academic competence, service resource utilisation and adverse events. Results A total of 250 children and young people from 98 schools were randomised: 127 to the intervention arm and 123 to the control arm. Intention-to-treat analysis of the main outcome measure showed a modest positive difference of 3.74 points (95% confidence interval –0.16 to 7.63 points, standardised effect size 0.18; p = 0.06) in favour of the intervention arm. Statistical significance increased in per-protocol analysis, with a modest positive difference (standardised effect size 0.21; p = 0.036). Cost-effectiveness of the intervention was found in reduced service use costs and a small increase in quality-adjusted life-years. Intervention fidelity and acceptability were positive. No intervention-related adverse events or effects were reported. Conclusions The primary and pre-planned sensitivity analysis of the primary outcome consistently showed a positive clinical difference, with modest standardised effect sizes of between 0.15 and 0.21. There were positive health economics and qualitative findings, corroborated by the difference between arms for the majority of secondary outcomes, which were not statistically significant but favoured the intervention arm. Post hoc additional analysis was exploratory and was not used in drawing this conclusion. Given the small positive change, LEGO^® based therapy for children and young people with autism spectrum disorder in schools should be considered. Limitations The primary outcome measure was completed by an unblinded teacher (rather than by the facilitator). Future work The study team recommends future research into LEGO^® based therapy, particularly in school environments. Trial registration This trial is registered as ISRCTN64852382. Funding This award was funded by the National Institute for Health and Care Research (NIHR) Public Health Research programme (NIHR award ref: 15/49/32) and is published in full in Public Health Research; Vol. 11, No. 12. See the NIHR Funding and Awards website for further award information.