BACKGROUND: Differences in the way autistic children experience the world can contribute to anxiety and stress. Carol Gray's Social Stories™ are a highly personalised intervention to support children by providing social information about specific situations in an individual story. OBJECTIVES: This randomised controlled trial aimed to establish whether Social Stories are clinically effective and cost-effective in improving social responsiveness and social and emotional health in children on the autism spectrum in schools. DESIGN: A multisite pragmatic cluster randomised controlled trial comparing Social Stories with care as usual. SETTING: Eighty-seven schools (clusters) across Yorkshire and the Humber. PARTICIPANTS: Two hundred and forty-nine children were randomised via a bespoke system hosted at York Trials Unit (129 Social Stories and 120 care as usual). Recruitment was completed in May 2021. Participants were children aged 4-11 years with a diagnosis of autism, alongside teachers, interventionists and caregivers. Recruitment was via schools, NHS trusts, support groups and local publicity. INTERVENTION: The intervention included training for educational professionals and caregivers covering psychoeducation and implementation of Social Stories. Stories were written around contextualised goals around the child's need for social information. Interventionists read the Social Story™ with the child at least six times over 4 weeks during school. MAIN OUTCOME MEASURE: The primary outcome was the Social Responsiveness Scale-2 completed by teachers at 6 months (the primary end point), which measures social awareness, cognition, communication and behaviour. Data were collected from caregivers and educational professionals at 6 weeks and 6 months through questionnaires. Blinding of participants was not possible. RESULTS: At 6 months, the estimated difference in expected teacher-reported Social Responsiveness Scale-2 T-score (the primary end point) was -1.61 (95% confidence interval -4.18 to 0.96,  = 0.220), slightly favouring the intervention group. The estimated differences for the parent-reported secondary outcomes at 6 months were small and generally favoured the control group except the measure of children's quality-adjusted life-year (+ 0.001, 95% confidence interval -0.032 to 0.035) and parental stress (-1.49, 95% confidence interval -5.43 to 2.46,  = 0.460), which favoured the intervention group. Children in the intervention group met their individual goals more frequently than children who received usual care alone (0.97 confidence interval 0.21 to 1.73,  = 0.012). The intervention is likely to save small costs (-£191 per child, 95% confidence interval -767.7 to 337.7) and maintain a similar quality of life compared to usual care. The probability of Social Stories being a preferred option is 75% if the society is willing to pay £20,000 per quality-adjusted life-year gained. Limitations include considerable disruptions during the coronavirus disease 2019 pandemic. CONCLUSION: Social Stories are used in schools and represent a low-cost intervention. There is no clinically evident impact on social responsiveness, anxiety and/or depression, parental stress or general health. Benefits were observed for specific behavioural goals as assessed by the teacher, and Social Stories may serve as a useful tool for facilitating dialogue between children and school staff to address specific behavioural challenges. Usage should be at the school's discretion. FUTURE WORK: Given the uncertainty of the results in light of coronavirus disease 2019, further work to establish the impact of Social Stories is merited. TRIAL REGISTRATION: This trial is registered as ISRCTN11634810. FUNDING: This award was funded by the National Institute for Health and Care Research (NIHR) Health Technology Assessment programme (NIHR award ref: 16/111/91) and is published in full in ; Vol. 28, No. 39. See the NIHR Funding and Awards website for further award information.