<jats:sec><jats:title>Background</jats:title><jats:p>Most people with mental disorders in communities exposed to adversity in low-income and middle-income countries (LMICs) do not receive effective care. Digital mental health interventions are scalable when digital access is adequate, and can be safely delivered during the COVID-19 pandemic.</jats:p></jats:sec><jats:sec><jats:title>Objective</jats:title><jats:p>To examine the effects of a new WHO-guided digital mental health intervention, Step-by-Step, supported by a non-specialist helper in Lebanon, in the context of concurring economic, humanitarian and political crises, a large industrial disaster and the COVID-19 pandemic.</jats:p></jats:sec><jats:sec><jats:title>Methods</jats:title><jats:p>We conducted a single-blind, two-arm pragmatic randomised trial, comparing guided Step-by-Step with enhanced care as usual (ECAU) among people suffering from depression and impaired functioning. Primary outcomes were depression (Patient Health Questionnaire 9 (PHQ-9)) and impaired functioning (WHO Disability Assessment Schedule-12 (WHODAS)) at post-treatment.</jats:p></jats:sec><jats:sec><jats:title>Findings</jats:title><jats:p>680 people with depression (PHQ-9&gt;10) and impaired functioning (WHODAS&gt;16) were randomised to Step-by-Step or ECAU. Intention-to-treat analyses showed effects on depression (standardised mean differences, SMD: 0.71; 95% CI: 0.45 to 0.97), impaired functioning (SMD: 0.43; 95% CI: 0.21 to 0.65), post-traumatic stress (SMD: 0.53; 95% CI: 0.27 to 0.79), anxiety (SMD: 0.74; 95% CI: 0.49 to 0.99), subjective well-being (SMD: 0.37; 95% CI: 0.12 to 0.62) and self-identified personal problems (SMD: 0.56; 95% CI 0.29 to 0.83). Significant effects on all outcomes were retained at 3-month follow-up.</jats:p></jats:sec><jats:sec><jats:title>Conclusions</jats:title><jats:p>Guided digital mental health interventions can be effective in the treatment of depression in communities exposed to adversities in LMICs, although some uncertainty remains because of high attrition.</jats:p></jats:sec><jats:sec><jats:title>Clinical implications</jats:title><jats:p>Guided digital mental health interventions should be considered for implementation in LMICs.</jats:p></jats:sec><jats:sec><jats:title>Trial registration number</jats:title><jats:p>ClinicalTrials.gov <jats:ext-link xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="clintrialgov" xlink:href="NCT03720769">NCT03720769</jats:ext-link>.</jats:p></jats:sec>