BACKGROUND: Because of wars, conflicts, persecutions, human rights violations, and humanitarian crises, about 84 million people are forcibly displaced around the world; the great majority of them live in low- and middle-income countries (LMICs). People living in humanitarian settings are affected by a constellation of stressors that threaten their mental health. Psychosocial interventions for people affected by humanitarian crises may be helpful to promote positive aspects of mental health, such as mental well-being, psychosocial functioning, coping, and quality of life. Previous reviews have focused on treatment and mixed promotion and prevention interventions. In this review, we focused on promotion of positive aspects of mental health. OBJECTIVES: To assess the effects of psychosocial interventions aimed at promoting mental health versus control conditions (no intervention, intervention as usual, or waiting list) in people living in LMICs affected by humanitarian crises. SEARCH METHODS: We searched CENTRAL, MEDLINE, Embase, and seven other databases to January 2023. We also searched the World Health Organization's (WHO) International Clinical Trials Registry Platform and ClinicalTrials.gov to identify unpublished or ongoing studies, and checked the reference lists of relevant studies and reviews. SELECTION CRITERIA: Randomised controlled trials (RCTs) comparing psychosocial interventions versus control conditions (no intervention, intervention as usual, or waiting list) to promote positive aspects of mental health in adults and children living in LMICs affected by humanitarian crises. We excluded studies that enrolled participants based on a positive diagnosis of mental disorder (or based on a proxy of scoring above a cut-off score on a screening measure). DATA COLLECTION AND ANALYSIS: We used standard Cochrane methods. Our primary outcomes were mental well-being, functioning, quality of life, resilience, coping, hope, and prosocial behaviour. The secondary outcome was acceptability, defined as the number of participants who dropped out of the trial for any reason. We used GRADE to assess the certainty of evidence for the outcomes of mental well-being, functioning, and prosocial behaviour. MAIN RESULTS: We included 13 RCTs with 7917 participants. Nine RCTs were conducted on children/adolescents, and four on adults. All included interventions were delivered to groups of participants, mainly by paraprofessionals. Paraprofessional is defined as an individual who is not a mental or behavioural health service professional, but works at the first stage of contact with people who are seeking mental health care. Four RCTs were carried out in Lebanon; two in India; and single RCTs in the Democratic Republic of the Congo, Jordan, Haiti, Bosnia and Herzegovina, the occupied Palestinian Territories (oPT), Nepal, and Tanzania. The mean study duration was 18 weeks (minimum 10, maximum 32 weeks). Trials were generally funded by grants from academic institutions or non-governmental organisations. For children and adolescents, there was no clear difference between psychosocial interventions and control conditions in improving mental well-being and prosocial behaviour at study endpoint (mental well-being: standardised mean difference (SMD) 0.06, 95% confidence interval (CI) -0.17 to 0.29; 3 RCTs, 3378 participants; very low-certainty evidence; prosocial behaviour: SMD -0.25, 95% CI -0.60 to 0.10; 5 RCTs, 1633 participants; low-certainty evidence), or at medium-term follow-up (mental well-being: mean difference (MD) -0.70, 95% CI -2.39 to 0.99; 1 RCT, 258 participants; prosocial behaviour: SMD -0.48, 95% CI -1.80 to 0.83; 2 RCT, 483 participants; both very low-certainty evidence). Interventions may improve functioning (MD -2.18, 95% CI -3.86 to -0.50; 1 RCT, 183 participants), with sustained effects at follow-up (MD -3.33, 95% CI -5.03 to -1.63; 1 RCT, 183 participants), but evidence is very uncertain as the data came from one RCT (both very low-certainty evidence). Psychosocial interventions may improve mental well-being slightly in adults at study endpoint (SMD -0.29, 95% CI -0.44 to -0.14; 3 RCTs, 674 participants; low-certainty evidence), but they may have little to no effect at follow-up, as the evidence is uncertain and future RCTs might either confirm or disprove this finding. No RCTs measured the outcomes of functioning and prosocial behaviour in adults. AUTHORS' CONCLUSIONS: To date, there is scant and inconclusive randomised evidence on the potential benefits of psychological and social interventions to promote mental health in people living in LMICs affected by humanitarian crises. Confidence in the findings is hampered by the scarcity of studies included in the review, the small number of participants analysed, the risk of bias in the studies, and the substantial level of heterogeneity. Evidence on the efficacy of interventions on positive mental health outcomes is too scant to determine firm practice and policy implications. This review has identified a large gap between what is known and what still needs to be addressed in the research area of mental health promotion in humanitarian settings.