Integration of mental health and psychosocial support (MHPSS) into primary health care in the Gaza Strip involves the delivery of mental health services within the essential service package provided by UNRWA, as recommended by WHO to fill the treatment gap. In early 2016, a successful pilot demonstrated that the integration is achievable in limited resource settings, and resulted in the adoption of a stepped care model to screen, identify, support, and treat patients. This study aimed to evaluate the MHPSS integration process and outcomes at UNRWA health centres in the Gaza Strip. We used a descriptive analytical study design, with quantitative and qualitative data collected from records, and individual feedback from patients and health care providers (HCPs). The study was performed in the 22 UNRWA health centres of the Gaza Strip and all participants agreed to participate either verbally or by written consent. Approval was also obtained from the UNRWA Health Program Gaza Field Office. The integration process started with the comprehensive training of 460 medical and nursing HCPs, in accordance with the Mental Health Gap Action Programme (mhGAP) Intervention Guide. On completion of training, the HCPs put into practice their learning, under close supervision for the first month and with only technical support thereafter. During the period of technical support, MHPSS activities, referral pathways, and proper reporting were ensured and standardised in all health centres. Overall, the integration process achieved success in training, implementation, and supervision. The success of training was assessed with an evaluation form and participants' feedback, the success of implementation by the numbers of people who benefitted, their response to the intervention, and their feedback, and the success of supervision by the standardisation of the process and the technical validity of the intervention. By the end of September, 2018, 80 857 high-risk patients were screened using the General Health Questionnaire 12 (GHQ-12) and 9·0% (7327) were identified as requiring intervention and support, and were assisted using the UNRWA stepped care approach. Of the individuals who were assisted, 30·2% (2212 of 7327) showed significant improvement and 38·9% (2849) were identified as having mental health issues according to the mhGAP. In the same period, 1020 patients benefited from 113 support groups, with excellent effects on their physical and mental health as assessed with qualitative data, patients' testimonies and, in some cases, laboratory investigations. The challenges that were faced were due to manual documentation and reporting, sociopolitical uncertainty, an increase in the number and needs of patients, and the imposed burden on HCPs. In addition, during implementation many individuals who were screened and identified required an urgent response: individuals who were suicidal and individuals whose mental health had been affected by the Great March of Return. The integration showed remarkable success in a short time as a result of the collective effort of all participants. However, there are challenges to overcome, and observation, further evaluation, and refinement of the process are needed to stabilise and sustain the integration of MHPSS services into primary health care for maximum benefit. WHO, and UNRWA resources. Copyright © 2021 Elsevier Ltd. All rights reserved.