Objective: The aim of the study was to synthesize the evidence of parent training (PT) as an early intervention for preschool children aged 2.5 to 6 years with ADHD or ADHD symptoms. Method: A systematic review and meta-analysis was conducted. Results: Sixteen studies including 1,003 children were analyzed. Parent-rated outcomes revealed moderate effect sizes (ESs; Hedges’ g) of 0.51 for ADHD symptoms, 0.4 for conduct problems, and 0.63 for negative parenting. Based on independent assessment, results were only significant for negative parenting. Parent-rated outcomes were sustained at follow-ups of 3 to 12 months. Program type, intervention modality, and child diagnostic status did not moderate the effect. Conclusion: PT was partially supported as an efficacious intervention for preschool children with ADHD or ADHD symptoms with moderate ESs on parent-rated outcomes, but no significant results on independently assessed ADHD symptoms.