BACKGROUND: Published by the World Health Organization (WHO) and United Nations High Commissioner for Refugees (UNHCR) in 2015, the mental health Gap Action Programme Humanitarian Intervention Guide (mhGAP-HIG) recommends brief versions of structured psychological interventions for people experiencing symptoms of common mental disorders (CMDs). mhGAP-HIG acknowledges a growing body of evidence suggesting these interventions can be delivered by lay workers to people affected by humanitarian crises in low- and middle-income countries (LMICs). However, there has not yet been a systematic review and synthesis of this evidence. This paper reports the results of a systematic review of qualitative, quantitative, and mixed-methods studies assessing the implementation and/or effectiveness of talk therapies for CMDs when provided by lay workers in LMICs to adults who have survived or are currently living in humanitarian situations. METHODS: Seven electronic databases were searched: MEDLINE, Embase, PsycINFO, PsycEXTRA, Global Health, Cochrane Library, and ClinicalTrials.gov . We also hand-searched the contents pages of three academic journals, reference lists of 30 systematic reviews, and online resource directories of two mental health networks. A preliminary list of included studies was circulated to topical experts for review, and all included studies were backward and forward searched. All titles, abstracts, and full-texts were independently double-screened. Quality appraisal and data extraction were carried out by a single reviewer and checked by a second reviewer, using standardised tools. Any disagreements were discussed and referred to a third reviewer as needed. RESULTS: We identified 23 unique studies and carried out a narrative synthesis of patient and implementation outcome data. Every evaluation of the effectiveness of lay-delivered talk therapies for adults affected by humanitarian crises in LMICs showed some treatment effect for at least one CMD, and often multiple CMDs. Implementation research generally found these interventions to be acceptable, appropriate and feasible to implement, with good fidelity to manualised therapies. CONCLUSION: Although results are promising, particularly for individually-delivered talk therapies based on cognitive behavioural therapy techniques, there is a high degree of heterogeneity in this literature. We make several recommendations on how to improve the quality and generalisability of research on this topic, to facilitate further evidence synthesis. TRIAL REGISTRATION: PROSPERO registration number: CRD42017058287 .