Orphans and vulnerable children (OVC) are at high risk for experiencing trauma and related psychosocial problems. Despite this, no randomized clinical trials have studied evidence-based treatments for OVC in low-resource settings. The objective of this study was to evaluate the effectiveness of lay counselor-provided trauma-focused cognitive behavioral therapy (TF-CBT) to address trauma and stress-related symptoms among OVC in Lusaka, Zambia. This randomized clinical trial compared TF-CBT and treatment as usual (TAU) (varying by site) for children recruited from August 1, 2012, through July 31, 2013, and treated until December 31, 2013, for trauma-related symptoms from 5 community sites within Lusaka, Zambia. Children were aged 5 through 18 years and had experienced at least one traumatic event and reported significant trauma-related symptoms. Analysis was with intent to treat. The intervention group received 10 to 16 sessions of TF-CBT (n = 131). The TAU group (n = 126) received usual community services offered to OVC. The primary outcome was mean item change in trauma and stress-related symptoms using a locally validated version of the UCLA Posttraumatic Stress Disorder Reaction Index (range, 0-4) and functional impairment using a locally developed measure (range, 0-4). Outcomes were measured at baseline and within 1 month after treatment completion or after a waiting period of approximately 4.5 months after baseline for TAU. At follow-up, the mean item change in trauma symptom score was -1.54 (95% CI, -1.81 to -1.27), a reduction of 81.9%, for the TF-CBT group and -0.37 (95% CI, -0.57 to -0.17), a reduction of 21.1%, for the TAU group. The mean item change for functioning was -0.76 (95% CI, -0.98 to -0.54), a reduction of 89.4%, and -0.54 (95% CI, -0.80 to -0.29), a reduction of 68.3%, for the TF-CBT and TAU groups, respectively. The difference in change between groups was statistically significant for both outcomes (P < .001). The effect size (Cohen d) was 2.39 for trauma symptoms and 0.34 for functioning. Lay counselors participated in supervision and assessed whether the intervention was provided with fidelity in all 5 community settings. The TF-CBT adapted for Zambia substantially decreased trauma and stress-related symptoms and produced a smaller improvement in functional impairment among OVC having experienced high levels of trauma.