INTRODUCTION: Non-adherence to antiretroviral therapy (ART) is the main cause of viral non-suppression and its risk is increased by depression. In countries with high burden of HIV, there is a lack of trained professionals to deliver depression treatments. This paper describes the protocol for a 2-arm parallel group superiority 1:1 randomised controlled trial, to test the effectiveness and cost effectiveness of the TENDAI stepped care task-shifted intervention for depression, ART non-adherence and HIV viral suppression delivered by lay interventionists. METHODS AND ANALYSIS: Two hundred and ninety people living with HIV aged ≥18 years with probable depression (Patient Health Questionnaire=>10) and viral non-suppression (≥ 1000 HIV copies/mL) are being recruited from HIV clinics in towns in Zimbabwe. The intervention group will receive a culturally adapted 6-session psychological treatment, Problem-Solving Therapy for Adherence and Depression (PST-AD), including problem-solving therapy, positive activity scheduling, skills to cope with stress and poor sleep and content to target barriers to non-adherence to ART. Participants whose score on the Patient Health Questionnaire-9 remains ≥10, and/or falls by less than 5 points, step up to a nurse evaluation for possible antidepressant medication. The control group receives usual care for viral non-suppression, consisting of three sessions of adherence counselling from existing clinic staff, and enhanced usual care for depression in line with the WHO Mental Health Gap intervention guide. The primary outcome is viral suppression (<1000 HIV copies/mL) at 12 months post-randomisation. ETHICS AND DISSEMINATION: The study and its tools were approved by MRCZ/A/2390 in Zimbabwe and RESCM-18/19-5580 in the UK. Study findings will be shared through the community advisory group, conferences and open access publications. TRIAL REGISTRATION NUMBER: NCT04018391.