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Introduction Global increases in armed conflict, forced displacement, pandemics and economic instability have contributed to rising levels of psychological distress worldwide, placing relevant segments of the population at increased risk of developing mental health conditions. This burden is particularly pronounced in humanitarian and low-resource settings where access to specialist mental health services is limited. Scalable, low-intensity, evidence-based psychological interventions are therefore urgently needed. In response, the WHO has developed transdiagnostic programmes, including Self-Help Plus (SH+) and Doing What Matters in Times of Stress (DWMS). Although these interventions are increasingly implemented across humanitarian and public health contexts, evidence for their effectiveness and implementation has not yet been systematically synthesised.Methods and analysis This preregistered systematic review and meta-analysis will be conducted in accordance with Cochrane Collaboration standards and reported following the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines. We will include randomised controlled trials evaluating the effectiveness of SH+ or DWMS, alongside qualitative and mixed-methods studies examining their implementation among stressor-exposed individuals of any age. Outcomes will include symptoms of depression and anxiety, general distress and post-traumatic stress symptoms. Moreover, we will examine effects on well-being, psychosocial functioning, adverse events and implementation outcomes (eg, acceptability, feasibility, fidelity). We will search Cochrane CENTRAL, APA PsycNet, Web of Science Core Collection, Embase and Scopus for records published from 2016 onwards. Searches will be supplemented by hand-searching preprint repositories and citation tracking. Risk of bias will be assessed using the Revised Cochrane Risk of Bias Tool and a customised appraisal tool for studies on implementation. Quantitative data will be synthesised using random-effects multilevel meta-analyses, with meta-regression models applied to examine moderators. Bayesian meta-analyses will be conducted where appropriate as sensitivity analyses to assess the robustness of the findings. Certainty of evidence will be evaluated using the Grading of Recommendations, Assessment, Development and Evaluation (GRADE) approach.Ethics and dissemination Ethical approval is not required. Findings will be disseminated through an open-access peer-reviewed publication, a plain-language summary, and the Open Science Framework, where all materials will be made publicly available.PROSPERO registration number CRD420251168521.