Background: Studies of scalable psychological interventions in humanitarian setting are usually carried out when the acute emergency has stabilized. We report the first evaluation of an evidence-based group psychological intervention, Group Integrative Adapt Therapy (IAT-G), during the emergency phase of a mass humanitarian crisis amongst Rohingya refugees in Cox's Bazar, Bangladesh. Methods: We did a pragmatic naturalistic evaluation (2018-2020) of a seven-session group intervention with adult Rohingya refugees with elevated symptoms of depression (≥10 on the Patient Health Questionnaire) and/or posttraumatic stress disorder, PTSD, (≥3 on the Posttraumatic Stress Disorder-8), and functional impairment (≥17 on WHO Disability Assessment Schedule or WHODAS-brief). Screening was done across the most densely populated campsites. Blind assessments were completed at baseline, posttreatment, and at 3-month follow-up using culturally adapted measures of depression, anxiety, posttraumatic stress symptoms, complicated bereavement, adaptive stress associated with disrupted psychosocial support systems, functional impairment, and resilience. Findings: 383 persons were screened and of the 144 persons who met inclusion criteria all participated in the group intervention. Compared to baseline scores, IAT-G participants recorded significantly lower mean scores on key outcome indices (mental health symptoms, adaptive stress, and functional impairment) at posttreatment and 3-month follow-up (all pairwise tests significant Ps