Objective: A community-based randomized controlled trial (RCT) was conducted in urban areas characterized by high levels of disadvantage to test the effectiveness of the Incredible Years BASIC parent training program (IYBP) for children with behavioral problems. Potential moderators of intervention effects on child behavioral outcomes were also explored. Method: Families were included if the child (aged 32–88 months) scored above a clinical cutoff on the Eyberg Child Behavior Inventory (ECBI). Participants (n = 149) were randomly allocated on a 2:1 ratio to an intervention group (n = 103) or a waiting-list control group (n = 46). Child behavior, parenting skills, and parent well-being were assessed at baseline and 6 months later using parent-report and independent observations. An intentionto- treat analysis of covariance was used to examine postintervention differences between groups. Results: Statistically significant differences in child disordered behavior favored the intervention group on the ECBI Intensity (effect size = 0.7, p = .001) and Problem subscales (effect size = 0.75, p = .001). Intervention effects on child hyperactive-inattentive behaviors and social competence, as well as parent competencies and well-being, were also found. Moderator analyses showed that the effects of the IYBP intervention on the primary child outcomes were not moderated by child or family demographic characteristics or risk factors. Conclusion: The results demonstrate the effectiveness of the IYBP in alleviating problem behavior among children and in improving well-being among families living in disadvantaged areas. The findings also highlight the importance of parental intervention in early childhood for parents and children most in need of support.