The literature provides solid documentation of the negative consequences of poor psychosocial functioning and behavioral problems in early childhood. Children facing economic disadvantage are at higher risk for poor socio-emotional development and problem behaviors, which draws attention to the need for prevention targeted at this risk group. The aim of our study was to analyze the impact of an evidence-based teacher-training program, the Incredible Years Teacher Classroom Management (IY-TCM), on the behavior of economically disadvantaged Portuguese children. Participants were 1030 children aged 3–6 years, from 65 preschool classrooms selected for their high percentage of children from economically disadvantaged backgrounds. The study followed an experimental randomized controlled between-group design, with pre- and post-intervention assessments. Teachers in the experimental group attended the IY-TCM program, implemented in six monthly workshops, and four sessions of individual in-class support. Children outcomes were evaluated with the Preschool and Kindergarten Behavior Scales—2nd Edition (Social Skills and Problem Behavior scales). After the intervention, preschoolers in the IY-TCM classrooms showed more improvements in their social skills and more decreases in problem behavior. Children with lower levels of social skills (high risk) at pre-intervention and those coming from families in economic need showed greater improvements in social skills, but these effects were not observed for problem behavior. Results support the growing evidence of the effectiveness of the IY-TCM as a stand-alone, non-adapted program in a Portuguese cultural context, which can play a protective role in children’s lives and thus bridge the gap between children coming from different economic backgrounds.