Background Falls in care home residents are common, unpleasant, costly and difficult to prevent. Objectives The objectives were to evaluate the clinical effectiveness and cost-effectiveness of the Guide to Action for falls prevention in Care Homes (GtACH) programme. Design A multicentre, cluster, parallel, 1 : 1 randomised controlled trial with embedded process evaluation and economic evaluation. Care homes were randomised on a 1 : 1 basis to the GtACH programme or usual care using a secure web-based randomisation service. Research assistants, participating residents and staff informants were blind to allocation at recruitment; research assistants were blind to allocation at follow-up. NHS Digital data were extracted blindly. Setting Older people’s care homes from 10 UK sites. Participants Older care home residents. Intervention The GtACH programme, which includes care home staff training, systematic use of a multidomain decision support tool and implementation of falls prevention actions, compared to usual falls prevention care. Outcomes The primary trial outcome was the rate of falls per participating resident occurring during the 90-day period between 91 and 180 days post randomisation. The primary outcome for the cost-effectiveness analysis was the cost per fall averted, and the primary outcome for the cost–utility analysis was the incremental cost per quality adjusted life-year. Secondary outcomes included the rate of falls over days 0–90 and 181–360 post randomisation, activity levels, dependency and fractures. The number of falls per resident was compared between arms using a negative binomial regression model (generalised estimating equation). Results A total of 84 care homes were randomised: 39 to the GtACH arm and 45 to the control arm. A total of 1657 residents consented and provided baseline measures (mean age 85 years, 32% men). GtACH programme training was delivered to 1051 staff (71% of eligible staff) over 146 group sessions. Primary outcome data were available for 630 GtACH participants and 712 control participants. The primary outcome result showed an unadjusted incidence rate ratio of 0.57 (95% CI 0.45 to 0.71; p < 0.01) in favour of the GtACH programme. Falls rates were lower in the GtACH arm in the period 0–90 days. There were no other differences between arms in the secondary outcomes. Care home staff valued the training, systematic strategies and specialist peer support, but the incorporation of the GtACH programme documentation into routine care home practice was limited. No adverse events were recorded. The incremental cost was £20,889.42 per Dementia Specific Quality of Life-based quality-adjusted life-year and £4543.69 per quality-adjusted life-year based on the EuroQol-5 dimensions, five-level version. The mean number of falls was 1.889 (standard deviation 3.662) in the GtACH arm and 2.747 (standard deviation 7.414) in the control arm. Therefore, 0.858 falls were averted. The base-case incremental cost per fall averted was £190.62. Conclusion The GtACH programme significantly reduced the falls rate in the study care homes without restricting residents’ activity levels or increasing their dependency, and was cost-effective at current thresholds in the NHS. Future work Future work should include a broad implementation programme, focusing on scale and sustainability of the GtACH programme. Limitations A key limitation was the fact that care home staff were not blinded, although risk was small because of the UK statutory requirement to record falls in care homes. Trial registration This trial is registered as ISRCTN34353836. Funding This project was funded by the National Institute for Health Research (NIHR) Health Technology Assessment programme and will be published in full in Health Technology Assessment; Vol. 26, No. 9. See the NIHR Journals Library website for further project information.