Background People living in 'humanitarian settings' in low‐ and middle‐income countries (LMICs) are exposed to a constellation of physical and psychological stressors that make them vulnerable to developing mental disorders. A range of psychological and social interventions have been implemented with the aim to prevent the onset of mental disorders and/or lower psychological distress in populations at risk, and it is not known whether interventions are effective. Objectives To compare the efficacy and acceptability of psychological and social interventions versus control conditions (wait list, treatment as usual, attention placebo, psychological placebo, or no treatment) aimed at preventing the onset of non‐psychotic mental disorders in people living in LMICs affected by humanitarian crises. Search methods We searched the Cochrane Common Mental Disorders Controlled Trials Register (CCMD‐CTR), the Cochrane Drugs and Alcohol Review Group (CDAG) Specialized Register, Cochrane Central Register of Controlled Trials (CENTRAL), MEDLINE (OVID), Embase (OVID), PsycINFO (OVID), and ProQuest PILOTS database with results incorporated from searches to February 2020. We also searched the World Health Organization's (WHO) International Clinical Trials Registry Platform and ClinicalTrials.gov to identify unpublished or ongoing studies. We checked the reference lists of relevant studies and reviews. Selection criteria All randomised controlled trials (RCTs) comparing psychological and social interventions versus control conditions to prevent the onset of mental disorders 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 calculated standardised mean differences for continuous outcomes and risk ratios for dichotomous data, using a random‐effects model. We analysed data at endpoint (zero to four weeks after therapy) and at medium term (one to four months after intervention). No data were available at long term (six months or longer). We used GRADE to assess the quality of evidence. Main results In the present review we included seven RCTs with a total of 2398 participants, coming from both children/adolescents (five RCTs), and adults (two RCTs). Together, the seven RCTs compared six different psychosocial interventions against a control comparator (waiting list in all studies). All the interventions were delivered by paraprofessionals and, with the exception of one study, delivered at a group level. None of the included studies provided data on the efficacy of interventions to prevent the onset of mental disorders (incidence). For the primary outcome of acceptability, there may be no evidence of a difference between psychological and social interventions and control at endpoint for children and adolescents (RR 0.93, 95% CI 0.78 to 1.10; 5 studies, 1372 participants; low‐quality evidence) or adults (RR 0.96, 95% CI 0.61 to 1.50; 2 studies, 767 participants; very low quality evidence). No information on adverse events related to the interventions was available. For children's and adolescents' secondary outcomes of prevention interventions, there may be no evidence of a difference between psychological and social intervention groups and control groups for reducing PTSD symptoms (standardised mean difference (SMD) −0.16, 95% CI −0.50 to 0.18; 3 studies, 590 participants; very low quality evidence), depressive symptoms (SMD −0.01, 95% CI −0.29 to 0.31; 4 RCTs, 746 participants; very low quality evidence) and anxiety symptoms (SMD 0.11, 95% CI −0.09 to 0.31; 3 studies, 632 participants; very low quality evidence) at study endpoint. In adults' secondary outcomes of prevention interventions, psychological counselling may be effective for reducing depressive symptoms (MD −7.50, 95% CI −9.19 to −5.81; 1 study, 258 participants; very low quality evidence) and anxiety symptoms (MD −6.10, 95% CI −7.57 to −4.63; 1 study, 258 participants; very low quality evidence) at endpoint. No data were available for PTSD symptoms in the adult population. Owing to the small number of RCTs included in the present review, it was not possible to carry out neither sensitivity nor subgroup analyses. Authors' conclusions Of the seven prevention studies included in this review, none assessed whether prevention interventions reduced the incidence of mental disorders and there may be no evidence for any differences in acceptability. Additionally, for both child and adolescent populations and adult populations, a very small number of RCTs with low quality evidence on the review's secondary outcomes (changes in symptomatology at endpoint) did not suggest any beneficial effect for the studied prevention interventions. Confidence in the findings is hampered by the scarcity of prevention studies eligible for inclusion in the review, by risk of bias in the studies, and by substantial levels of heterogeneity. Moreover, it is possible that random error had a role in distorting results, and that a more thorough picture of the efficacy of prevention interventions will be provided by future studies. For this reason, prevention studies are urgently needed to assess the impact of interventions on the incidence of mental disorders in children and adults, with extended periods of follow‐up., Plain language summary Do psychological and social interventions prevent mental health disorders in low‐ and middle‐income countries affected by humanitarian crises? Mental health during a humanitarian crisis A humanitarian crisis is an event, or series of events, that threaten the health, safety, security or well‐being of a community or large group of people, usually over a wide area. Examples include: wars, famine, and natural disasters such as earthquakes, hurricanes and floods. People living through a humanitarian crisis may experience physical and mental distress that make them vulnerable to developing mental health disorders. These include post‐traumatic stress disorder, depression and anxiety. What are psychological and social interventions? Psychological interventions of a preventive nature usually offer people support and practical help to develop ways of coping, a sense of hope, and focus on building resilience. Social interventions of a preventive nature usually aim to strengthen social support systems and help people to feel more connected. Why we did this Cochrane Review We wanted to know if psychological and social interventions (psychosocial interventions) could help to stop mental health disorders developing in people living through humanitarian crises in low‐ and middle‐income countries. We were interested in: 1)·how many people developed a mental health disorder after taking part in an intervention; and 2) how many people dropped out of a programme or had unwanted effects related to the intervention. What did we do? We searched for studies that looked at the preventive effects of psychosocial interventions on people's mental health in low‐ and middle‐income countries affected by humanitarian crises. We looked for randomised controlled studies, in which the interventions people received were decided at random. This type of study usually gives the most reliable evidence about the effects of an intervention. Search date We included evidence published up to February 2020. What we found We found seven prevention studies with a total of 2398 participants. Five studies were in children and adolescents (aged 7 to 18 years), and two were in adults (aged over 18 years). Two studies were done in Nepal, and one study each in Democratic Republic of Congo, Haiti, Syria, Uganda and Sri Lanka. Six different psychosocial interventions were studied. The studies measured symptoms of depression, anxiety and post‐traumatic stress disorder in children and adolescents, and anxiety and depression symptoms in adults, at the beginning of the study, the end of the intervention, and after four weeks and up to four months later. They compared the results with symptoms measured in people on a waiting list to take part in the intervention. What are the results of our review? None of the studies measured how many people developed a mental disorder after taking part in a psychosocial intervention, and none measured any unwanted effects of the interventions. There may be little to no difference in how many children and adolescents dropped out of an intervention while taking part, compared with being on a waiting list (5 studies). We were uncertain if there was any difference in the number of adults who dropped out (2 studies). In children and adolescents, only very small differences in symptoms of post‐traumatic stress disorder, depression and anxiety were seen at the end of an intervention, compared with being on a waiting list, suggesting no evidence of a difference. However, we are not confident that these results are reliable: the results are likely to change when further evidence is available. In adults, results from one prevention study showed that psychological counselling may lower depression and anxiety symptoms; but this result is from only one study and we are not confident the result is reliable. This result will probably change when more evidence becomes available. Conclusions We did not find any randomized evidence whether psychosocial interventions can stop mental health disorders developing in people living through humanitarian crises in low‐ to middle‐income countries. We did not find enough reliable evidence about the benefits of these interventions in reducing mental health symptoms. Larger, well‐conducted studies are needed to give more reliable evidence about the short‐ and long‐term effects of psychosocial interventions to prevent mental disorders in people living in low‐ and middle‐income countries affected by humanitarian crises.