BACKGROUND: Strengthening the evidence base for humanitarian interventions that provide psychosocial support to war-affected youth is a key priority. We tested the impacts of an 8-week programme of structured activities informed by a profound stress attunement (PSA) framework (Advancing Adolescents), delivered in group-format to 12-18 year-olds in communities heavily affected by the Syrian crisis. We included both Syrian refugee and Jordanian youth. METHODS: We followed an experimental design, comparing treatment youth and wait-list controls over two programme implementation cycles, randomizing to study arm in cycle 2 (ClinicalTrials.gov ID: NCT03012451). We measured insecurity, distress, mental health difficulties, prosocial behaviour and post-traumatic stress symptoms at three time-points: baseline (n = 817 youth; 55% Syrian, 43% female), postintervention (n = 463; 54% Syrian, 47% female), and follow-up (n = 212, 58% Syrian, 43% female). Regression models assessed: prospective intervention impacts, adjusting for baseline scores, trauma exposure, age, and gender; differential impacts across levels of trauma exposure and activity-based modality; and sustained recovery 1 year later. We analysed cycle-specific and cycle-pooled data for youth exclusively engaged in Advancing Adolescents and for the intent-to-treat sample. RESULTS: We found medium to small effect sizes for all psychosocial outcomes, namely Human Insecurity (beta = -7.04 (95% CI: -10.90, -3.17), Cohen’s d = -0.4), Human Distress (beta = -5.78 (-9.02, -2.54), d = -0.3), and Perceived Stress (beta = -1.92 (-3.05, -0.79), d = -0.3); and two secondary mental health outcomes (AYMH: beta = -3.35 (-4.68, -2.02), d = -0.4; SDQ: beta = -1.46 (-2.42, -0.50), d = -0.2). We found no programme impacts for prosocial behaviour or post-traumatic stress reactions. Beneficial impacts were stronger for youth with exposure to four trauma events or more. While symptoms alleviated for both intervention and control groups over time, there were sustained effects of the intervention on Human Insecurity. CONCLUSIONS: Findings strengthen the evidence base for mental health and psychosocial programming for a generation affected by conflict and forced displacement. We discuss implications for programme implementation and evaluation research.