Objective: This randomized control trial evaluated the efficacy of an online self-help program in a sample of parents of preschoolers with ADHD symptoms. Method: Parents were randomly assigned to the intervention group (n = 27) or the delayed intervention group (n = 26). Child behavior measures were completed by mothers, fathers, and teachers, and parenting measures were completed by mothers. Results: Intent-to-treat analyses indicated significant post-intervention improvements in mother-rated child hyperactivity/inattention, restlessness/impulsivity, defiance/aggression, social functioning, and teacher-rated prosocial behavior, as well as significant improvements in maternal over-reactivity, verbosity, laxness, positive parenting, parenting satisfaction, self-efficacy, stress, and depression. At 6-month follow-up, effects were maintained for maternal over-reactivity and verbosity, parenting satisfaction and self-efficacy, and parental stress and depression. Conclusion: This study provides evidence for the effectiveness of an online self-help parenting program in reducing preschool inattentive behavior difficulties, and in increasing parenting competence, satisfaction in the parenting role, and maternal well-being.