Relative to parents of children with neurotypical development and other developmental disabilities, parents of autistic children experience higher levels of parenting stress, which are associated with deleterious consequences for parents' mental and physical health and child functioning. Despite urgent calls to action, parenting stress is rarely addressed directly in interventions for families of autistic children, and less so in underserved and racial/ethnic minority populations where clinical needs are greater. This study tested the efficacy of mindfulness-based stress reduction, compared to a psychoeducation and support intervention, in reducing parenting stress among diverse families of autistic preschoolers. Participants (N = 117) were randomly assigned to the mindfulness-based stress reduction or psychoeducation and support groups; assessments were conducted at baseline, immediately postintervention, and 6 and 12 months postintervention. Results indicated significant reductions in parenting stress across both the mindfulness-based stress reduction and psychoeducation and support intervention conditions; however, reductions in parenting stress were greater for parents in mindfulness-based stress reduction than in psychoeducation and support. Furthermore, the benefit of mindfulness-based stress reduction relative to psychoeducation and support increased over time, with significant group differences in parenting stress detected at 12-month follow-up.