Parent stress has been associated with negative outcomes for youth and may be particularly high during adolescence. Mindfulness interventions have the potential to reduce parent stress and to improve parenting behavior and parent-child relationship quality. The present randomized controlled study examined effects of a parenting-focused mindfulness intervention, the Parenting Mindfully (PM) intervention, for highly stressed parents of adolescents. Eighty-three mothers of 12- to 17-year olds reporting high stress were randomly assigned to the PM intervention or to a minimal intervention Parent Education (PE) control group. At pre- and post-intervention, mothers reported on their mindfulness, stress, parenting stress, mindful parenting, and parent-adolescent relationship quality. At pre- and post-intervention, mothers’ observed parenting behaviors and reported negative emotional responses to a laboratory parent-adolescent interaction task (PAIT) were also collected. Findings indicated that the PM intervention, compared to PE, increased mothers’ mindfulness, reduced parenting stress in two domains, increased mindful parenting related to emotional awareness in parenting, and improved parent-adolescent relationship quality. For mothers of girls (but not mothers of boys), the PM intervention also decreased negative parenting behavior and decreased negative emotional responses in PAIT. Effects sizes were medium to large. In sum, findings support parenting-focused mindfulness training as a viable intervention strategy for highly stressed parents.