This study aimed to investigate the factor structure and external correlates of the constructs Reactive Attachment Disorder (RAD) and Disinhibited Social Engagement Disorder (DSED) from the Diagnostic and Statistical Manual of Mental Disorders (DSM-5). The following were addressed: First, do our data support the DSM-5 conceptualization of RAD/DSED as two separate constructs? Second, are RAD and DSED distinct from other well-established dimensions of child psychopathology? Third, what are the external correlates of RAD/DSED in this sample? The study sample included 122 foster children aged 6-10 years. Foster parents completed the Strengths and Difficulties Questionnaire (SDQ), and the RAD/DSED-scale from the Developmental and Well-Being Assessment. Child protection caseworkers completed a questionnaire regarding exposure to maltreatment and placement history. Confirmatory factor analysis (CFA) of the RAD/DSED items identified a good fit for a model with a two-factor structure, which is congruent with the DSM-5 definition of RAD and DSED. A new CFA model, which included the RAD and DSED factors together with the four problem factors of the SDQ (emotional, conduct, hyperactivity-inattention, and peer problems), also demonstrated a good fit with our data. RAD and DSED were associated with the SDQ Impact scale and help seeking behavior. This was partly explained by the SDQ externalizing and peer problem subscales. Our findings lend support for the DSM-5 conceptualization of RAD and DSED as separate dimensions of child psychopathology. Thus, the assessment of RAD and DSED provides information beyond other mental health problems.