Objective In South Korea, the placement of orphans in institutions is still common, despite evidence of its adverse influence on children’s psychological, emotional, physical, and cognitive development. In this preliminary study, we evaluate whether system-wide positive behavior support (SWPBS) is effective for decreasing externalizing problems in institutionalized children. Methods SWPBS was provided for one month to 36 school-aged boys who had lived in an orphanage for an average of 8.72 years (SD=2.52), along with their 10 caregivers. Direct observation of the frequency of target behaviors in the participating children, caregivers’ ratings of the severity of children’s problematic behaviors, self-reported caregiving stress (i.e., Parenting Stress Scale), and intervention fidelity were dependent variables. Results After receiving SWPBS, the frequency of problematic behaviors in the participating children showed a mean decrease of 73.6%. The caregivers also reported seeing a decrease in the severity of their children’s problematic behaviors after SWPBS compared to before. Conclusion SWPBS may be a promising intervention to decrease externalizing behaviors in school-aged children who have been living in institutions for a long time and have shown mental health issues.