Objective: This study tested the effectiveness of Parent Management Training, Oregon (PMTO) model on child social-emotional well-being. Methods: Using a randomized controlled design and three measures of social-emotional well-being, the study investigated effectiveness of PMTO with families of children in foster care with serious emotional disturbance (SED). Participants included children (3-16 years) and parents who were randomly assigned to PMTO (n = 461) or services as usual (n = 457). Study condition was known to participants and assessors. Six months after baseline, analysis of covariance models examined the intervention's overall effect and time interactions using intent-to-treat analysis. Follow-up analyses identified salient predictors of well-being. Results: PMTO demonstrated small but significant positive effects on three primary outcomes: social-emotional functioning (Cohen's d = 0.31), problem behaviors (Cohen's d = 0.09), and prosocial skills (Cohen's d = 0.09). Conclusion: Results suggest that PMTO was effective at improving short-term social-emotional well-being in a high-risk population of children with SED.