Parents of autistic children often experience high levels of parenting stress, which can have negative mental and physical effects on both the parent and child. This study tested the efficacy of mindfulness-based stress reduction in reducing parenting stress in parents of preschool-aged autistic children compared to a psychoeducation and support intervention. We assessed parenting stress before and after the interventions and at 6- and 12-month follow-up. Both interventions significantly decreased parenting stress, but mindfulness-based stress reduction reduced stress more than did psychoeducation and support, with the strongest effect observed 1 year later. This suggests that the stress-reducing benefits of mindfulness-based stress reduction persist and may increase over time.