In 2019, the American Pediatric Association released updated guidelines regarding assessing and treating attention-deficit/hyperactivity disorder (ADHD). These guidelines prescribe behavioral parent training (BPT) as the first line of treatment for preschool-aged children with ADHD symptomatology. However, there are few studies documenting the effectiveness of BPT administered in real-word settings, outside the context of controlled research trials. Further, many available BPT programs are lengthy and costly. Here we investigate the effectiveness of an 8-session group-based BPT, previously shown effective in school-aged children with ADHD, in an outpatient clinic. Ratings of parental confidence, child impairment, number and severity of non-compliance in multiple situations, and ADHD and oppositional symptoms, were collected from 132 parents of preschoolers at risk for ADHD before and after the BPT groups. Improvements with a large effect size were observed for overall severity of impairment and parental confidence ratings. Improvements with a moderate effect size were observed for academic progress, severity of non-compliance, and ADHD symptom ratings. Small effect sizes were observed for reduced impairment in family functioning, peer and parental relationships, and self-esteem, frequency of non-compliance, and oppositional defiant symptom ratings. Together these findings support the effectiveness of a BPT program to treat preschool ADHD and ADHD-like behaviors that is accessible to families and clinicians.