The prevention of child maltreatment necessitates a public health approach. In the U.S. Triple P System Population Trial, 18 counties were randomly assigned to either dissemination of the Triple P-Positive Parenting Program system or to the services-as-usual control condition. Dissemination involved Triple P professional training for the existing workforce (over 600 service providers), as well as universal media and communication strategies. Large effect sizes were found for three independently derived population indicators: substantiated child maltreatment, child out-of-home placements, and child maltreatment injuries. This study is the first to randomize geographical areas and show preventive impact on child maltreatment at a population level using evidence-based parenting interventions.