Parents of children affected by developmental delays (DD) have been shown to experience elevated levels of parenting stress compared to parents of typically developing children. Recent studies suggest that higher levels of parenting stress can adversely impact child behavior, such that higher levels of parenting stress are often associated with higher levels of both internalizing and externalizing behavior problems. However, the specific mechanisms through which parenting stress influences child behavioral outcomes remain unclear. The parent-child relationship may be one potential medium through which parenting stress impacts child behavioral outcomes. The current study involved data from the Mindful Awareness for Parenting Stress Project, a randomized controlled trial examining the efficacy of mindfulness-based stress reduction in reducing parental stress and subsequent child behavior problems. Participants included 102 parents of children, ages 2.5 to 5 years old, with DD. Results of a multiple mediation analysis indicted that one component of the parent-child relationship, parent-child relational frustration, significantly mediated the relationship between parenting stress and child outcomes, such that higher levels of parental stress at baseline were associated with increased relational frustration, which in turn was associated with more total child behavior problems. Recognizing that dyadic parent-child factors such as relational frustration impact the association between heightened parenting stress and adverse behavioral outcomes among children with DD may allow interventions to be developed that not only reduce parenting stress, but also specifically target improving the parent-child relationship, with the ultimate goal of decreasing child behavior problems among children with DD.