BACKGROUND: Common mental disorders are frequently experienced by refugees. This study evaluates the impact of a brief, lay provider delivered group-based psychological intervention [Group Problem Management Plus (gPM+)] on the mental health of refugees in a camp, as well as on parenting behavior and children's mental health. METHODS AND FINDINGS: In this single-blind, parallel, randomized controlled trial, 410 adult Syrian refugees (300 females, 110 males) in Azraq Refugee Camp (Jordan) were identified through screening of psychological distress (≥16 on the Kessler Psychological Distress Scale) and impaired functioning (≥17 on the WHO Disability Assessment Schedule). Participants were randomly allocated to gPM+ or enhanced usual care (EUC) involving referral information for psychosocial services on a 1:1 ratio. Participants were aware of treatment allocation, but assessors were blinded to treatment condition. Primary outcomes were scores on the Hopkins Symptom Checklist-25 (HSCL; depression and anxiety scales) assessed at baseline, 6 weeks, and 3 months follow-up as the primary outcome time point. It was hypothesized that gPM+ would result in greater reductions of scores on the HSCL than EUC. Secondary outcomes were disability, posttraumatic stress, personally identified problems, prolonged grief, prodromal psychotic symptoms, parenting behavior, and children's mental health. Between October 15, 2019 and March 2, 2020, 624 refugees were screened for eligibility, 462 (74.0%) screened positive, of whom 204 were assigned to gPM+ and 206 to EUC. There were 168 (82.4%) participants in gPM+ and 189 (91.7%) in EUC assessed at follow-up. Intent-to-treat analyses indicated that at follow-up, participants in gPM+ showed greater reduction on HSCL depression scale than those receiving EUC (mean difference, 3.69 [95% CI 1.90 to 5.48], p = .001; effect size, 0.40). There was no difference between conditions in anxiety (mean difference -0.56, 95% CI -2.09 to 0.96; p = .47; effect size, -0.03). Relative to EUC, participants in gPM+ had greater reductions in severity of personally identified problems (mean difference 0.88, 95% CI 0.07 to 1.69; p = .03), and inconsistent disciplinary parenting (mean difference 1.54, 95% CI 1.03 to 2.05; p < .001). There were no significant differences between conditions for changes in PTSD, disability, grief, prodromal symptoms, or childhood mental health outcomes. Mediation analysis indicated the change in inconsistent disciplinary parenting was associated with reduced attentional (β = 0.11, SE .07; 95% CI .003 to .274) and internalizing (β = 0.08, SE .05; 95% CI .003 to 0.19) problems in children. No adverse events were attributable to the interventions or the trial. Major limitations included only one-quarter of participants being male, and measures of personally identified problems, grief, prodromal psychotic symptoms, inconsistent parenting behavior, and children's mental health have not been validated with Syrians. CONCLUSIONS: In camp-based Syrian refugees, a brief group behavioral intervention led to reduced depressive symptoms, personally identified problems, and disciplinary parenting compared to usual care, and this may have indirect benefits for refugees' children. The limited capacity of the intervention to reduce PTSD, disability, or children's psychological problems points to the need for development of more effective treatments for refugees in camp settings. TRIAL REGISTRATION: Prospectively registered at Australian and New Zealand Clinical Trials Registry: ACTRN12619001386123.