Research Findings: The Incredible Years Teacher Classroom Management (IY-TCM) program has shown efficacy in reducing behavioral problems and improving social skills among children experiencing poverty. However, quantitative studies that concentrate on its efficacy in relation to “proximal classroom outcomes” (e.g., classroom emotional and managerial interactions between teachers and children) and teacher psychological factors (e.g., self-efficacy and stress) are still needed. This study assesses the efficacy of the IY-TCM program with Portuguese teachers of economically disadvantaged preschool children. Measures included a) classroom observation of teacher behaviors and b) teacher self-reports of their practices, confidence, self-efficacy, and job-related stress associated with inappropriate student behaviors. A wait-list randomized controlled study was conducted with 65 female Portuguese teachers reporting pre- and post-intervention assessments. Moderation analyses were conducted to explore differential benefits of teacher, classroom and children’s variables. Significant differences favoring intervention teachers were found in teacher reports of their classroom practices (positive and inappropriate). Observational findings showed a positive intervention effect on the use of labeled praise but not on other teacher classroom behaviors. Participating in the program did not impact the teachers’ confidence/self-efficacy and job-related stress. Moderation analysis were found to be non-significant. Practice or Policy: Providing the IY-TCM program to early childhood teachers can contribute to an increase of their reported use and usefulness of effective classroom management practices and their use of labeled praise, thus contributing to the quality of early childhood education across a population of children experiencing poverty.