<p>Many parents experience elevated levels of stress, and parents of children with externalizing behavioral concerns tend to experience even higher levels of stress due to the transactional and bidirectional nature of child behavior and parent stress. The Family Adjustment and Adaptation Response (FAAR) model suggests that families work to balance demands with capabilities, which interact with family meanings, to achieve adjustment or adaptation. Thus, increasing capabilities is an important task for families experiencing stress. In this hybrid effectiveness-implementation study, we evaluated whether an online mindfulness-based stress reduction (MBSR) program with favorable results in the general adult population showed effectiveness for stressed parents of children with behavior problems during the COVID-19 pandemic. We also examined acceptability and participant engagement with the program. Participants included 38 mothers, fathers, and other primary caretakers of children ages two to ten years with behavioral concerns living across the United States. Parents engaged in the 4-module Be Mindful (MBSR) program for between 4 and 10 weeks. Results provided support for the Be Mindful intervention within this particular population: there were robust, statistically significant and clinically meaningful reductions in parent stress following completion of the intervention. Reductions in stress were maintained through one-month follow-up. Additionally, participants generally found the intervention to be acceptable and practiced the skills a moderate amount during the intervention phase. Open-ended and quantitative feedback provided information regarding barriers and facilitators to intervention use.</p>