Introduction: Children with developmental delays (DD), particularly autism spectrum disorders (ASD), are at high risk for increased levels of behavior problems and developing internalizing problems. While previous literature has highlighted the impact of parental stress on the development of behavioral problems broadly in children with DD, research on the relation between parenting stress and internalizing problems in children with ASD is scarce. Growing evidence has supported Mindfulness-Based Stress Reduction (MBSR) as an effective intervention for reducing stress among parents of children with DD. These reductions in parent stress are also associated with subsequent reductions in their children's behavior problem; however, to our knowledge, the effects of MBSR and reductions in parent stress on internalizing problems in children with DD have not been examined. Method: The current study utilized data from Mindful Awareness for Parenting Stress (MAPS) which consisted of families of 80 preschool children with developmental delays (DD) whose parents reported high levels of stress. Specifically, we investigated whether decreases in parenting stress as a result of engagement in a MBSR intervention would lead to reductions in internalizing behavior problems among children with DD, and whether this relation was moderated by the child's ASD status. Additionally, we examined whether individual increases in the mindful facets of acting with awareness and nonjudgment, from pre- to post- treatment, would lead to reductions in internalizing problems among children with DD, and whether this relation was moderated by the child's ASD status. Results: Children of parents in the MBSR treatment group had greater reductions in internalizing problems compared to children whose parents were in the waitlist-control group, "b"= -5.71, "p" <0.05. We also found that children of parents who reported greater increases in acting with awareness post-treatment had a greater reduction in internalizing problems, "b"= -2.57, "p"<0.05. Additionally, children whose parents had greater increases in nonjudgment post-treatment had a greater reduction in internalizing problems, "b"= -1.85, "p"<0.05. However, ASD status was not a significant moderator in either analysis, "ps" >0.05 Conclusions: These findings have important implications regarding the relation between parenting stress and internalizing problems in children with DD. This treatment offers a novel approach to treating comorbid internalizing problems in children with DD. Parents' use of MBSR provides a method of early intervention, which may ameliorate the development of internalizing problems over time. The current study helps to inform future parenting interventions by highlighting the importance of addressing specific facets of mindfulness in interventions which may lead to greater reductions in internalizing problems in vulnerable populations such as children with DD.