Youth with intellectual and developmental disabilities (IDD) experience high rates of emotional and behavior problems. The Nisonger Child Behavior Rating Form (NCBRF) is one of the few tools developed to assess these problems in this population. It consists of a 10-item Social Competence section and a 66-item Problem Behavior section. The goal of this study was to examine the factorial, criterion, and convergent validity of the NCBRF. Data from individuals aged five to 18 years (n = 399) were submitted to confirmatory factor analysis (CFA). In addition, scores of different clinical groups (disruptive behavior disorders, mood/anxiety disorders, taking psychotropic medicines) were compared to youth with no significant clinical concerns and associations with Developmental Behavior Checklist (DBC) scores were examined in a subsample of participants. Results of the CFA indicated that the Social Competence subscales better fit the data (RMSEA = 0.056), than the Problem Behavior subscales (RMSEA = 0.086). Comparisons across clinical groups and correlations with DBC scores were significant and in the expected directions. This study supports the use of the NCBRF in youth with IDD, though the factor structure of the Problem Behavior section may need to be revisited. (Contains 3 tables.)