The current study examined the feasibility and initial efficacy of the Summer Treatment Program for Pre-kindergarteners (STP-PreK) with 37 preschoolers with autism spectrum disorder (ASD) and externalizing behavior problems (EBP). Parents and teachers reported on children's behavior, social/adaptive skills, and self-regulation. Children completed a standardized achievement and executive functioning battery and an emotion knowledge task. The treatment was delivered with high levels of fidelity and was well received by families. Improvements were reported in parent-rated hyperactivity, inattention, aggression, and social and adaptive skills. Children also improved performance across academic achievement, emotion knowledge, and executive functioning and were rated by parents as having better executive functioning and emotion regulation. Findings highlight the initial efficacy of an established treatment in improving outcomes for preschoolers with ASD and EBP.