INTRODUCTION: While highly effective for reducing the risk of tuberculosis (TB) disease, TB preventive treatment (TPT) is underused among people living with HIV (PWH). We evaluated the effectiveness of a behavioural economics-based choice architecture approach to increase facility-level TPT prescribing to PWH in Malawi, Mozambique and Zimbabwe. METHODS: We conducted a cluster-randomised trial within the IMPAACT4TB 3HP rollout, in which 57 healthcare facilities were randomly assigned 1:1 to choice architecture (intervention) or standard TPT prescribing (control). The aim was to link TPT to antiretroviral therapy (ART) prescribing and to make TPT prescribing the default. The intervention was supported by clinician training and a default prescribing module built into the point-of-care HIV electronic medical record in Malawi and stickers placed in clients' clinical stationery in Mozambique and Zimbabwe. Data were collected in aggregate, and the primary outcome was the facility-level percentage of clients initiating ART who initiated TPT. The CAT study was registered with clinicaltrials.gov where it is listed as completed. RESULTS: Implementation occurred from October 2021 to September 2022 in Mozambique (20 facilities), April 2021 to March 2022 in Malawi (19 facilities) and June 2021 to May 2022 in Zimbabwe (18 facilities), for a total of 29 control arm and 28 choice architecture intervention arm facilities, respectively. Comparing control to intervention facilities, mean TPT prescribing to clients initiating ART was 70.9% vs 86.9% in Mozambique (difference: -16.0%; 95% CI: -38.3%, 6.3%; p=0.15), 56.5% vs 55.5% in Malawi (difference: 1.0%; 95% CI: -14.0%, 16.9%; p=0.89) and 56.2% vs 55.9% in Zimbabwe (difference: 0.2%; 95% CI: -25.2%, 25.8%; p=0.98). CONCLUSION: The choice architecture intervention did not overcome barriers to TPT prescribing. While the intervention may have led to an improvement in TPT prescribing in Mozambique, no differences were observed in the other countries. Further innovation is needed to ensure that all clients initiating ART are either prescribed TPT or started on anti-TB treatment, as appropriate. TRIAL REGISTRATION NUMBER: NCT04466293.