The current study explored cross-sectional relations between coparenting quality and child problem behaviors, as measured by parent report and direct observation, in families of school-aged children previously identified with a developmental delay in early childhood. Parents' reports of difficulty with coparenting problems predicted child problem behaviors. For primary caregivers, parenting self-efficacy mediated the relation between coparenting quality and problem behaviors. Observed undermining behavior significantly positively predicted child appropriate behavior across specific tasks and observed partner support behavior significantly negatively predicted child appropriate behavior across specific tasks. Discussion focuses on the clinical significance of these findings and future research directions.