Background: The Democratic Republic of Congo (DRC) has been home to the world's deadliest conflict since World War II and is reported to have the largest number of child soldiers in the world. Despite evidence of the debilitating impact of war, no group-based mental health or psychosocial intervention has been evaluated in a randomised controlled trial for psychologically distressed former child soldiers. Method: A randomised controlled trial involving 50 boys, aged 13-17, including former child soldiers (n = 39) and other war-affected boys (n = 11). They were randomly assigned to an intervention group, or wait-list control group. The intervention group received a 15-session, group-based, culturally adapted Trauma-Focused Cognitive-Behavioural Therapy (TF-CBT) intervention. Assessment interviews were completed at baseline, postintervention and 3-month follow-up (intervention group). Results: Analysis of Covariance (ANCOVA) demonstrated that, in comparison to the wait-list control group, the TF-CBT intervention group had highly significant reductions in posttraumatic stress symptoms, overall psychosocial distress, depression or anxiety-like symptoms, conduct problems and a significant increase in prosocial behaviour (p < 0.001 for all). Effect sizes were higher when former child soldier scores were separated for sub-analysis. Three-month follow-up of the intervention group found that treatment gains were maintained. Conclusions: A culturally modified, group-based TF-CBT intervention was effective in reducing posttraumatic stress and psychosocial distress in former child soldiers and other war-affected boys.