Incredible Years (IY) is a well-established multicomponent group-based program designed to promote young children's emotional and social competence, to prevent and treat child behavioral and emotional problems, and to improve parenting practices and the parent-child relationship. This study presents the first randomized controlled trial carried out in Spain to test the effectiveness of the Incredible Years Basic Parenting and Small Group Dinosaur Programs in a sample of families involved in child welfare due to substantiated or risk for child maltreatment. One hundred and eleven families with 4- to 8-year-old children were randomly allocated to IY or to a control group who received standard services. Baseline, post-intervention, and 12-month follow-up assessments were compared. Results showed that compared to the control group, the IY intervention made a significant positive difference in parents' observed and reported use of praise, and a significant reduction in reported use of inconsistent discipline, parenting stress, depressive symptomatology, and perception of child behavior problems. A full serial mediation effect was found between participation in IY, changes in parenting practices, subsequent parenting stress reduction, and both final child abuse potential reduction and perception of child behavior problems. No moderating influence on IY effects was found. Findings provide evidence that transporting the IY Basic Parenting and the Small Group Dinosaur Programs with fidelity is feasible in Child Welfare Services in Spain.