Among preschool children referred for psychological assessment and intervention, attention-deficit/hyperactivity disorder (ADHD) is the most commonly diagnosed disorder. In order to create school-based interventions that are consistently and effectively implemented by teachers, it is important to understand teachers' knowledge and beliefs about ADHD, as studies have identified lack of knowledge as a barrier to treatment integrity. The present study seeks to fill the important gaps of previous work and gain an understanding of preschool teachers' knowledge and opinions of ADHD, and how knowledge of ADHD impacts teachers' perceptions of child impairment across domains. The present study includes teachers (n = 107; 97.2% female; 59.2% Hispanic or Latino; 16.8% Black or African-American) of ethnically and linguistically diverse preschool students in an urban school setting who were being referred for a summer treatment program. Teachers' knowledge of ADHD was investigated and related to teacher characteristics (i.e., education, years of experience, classroom type, professional development) and ratings of child impairment across domains. Overall, preschool teachers achieved 38.3% accuracy on the knowledge of ADHD measure, with highest levels of inaccurate knowledge related to the symptoms and treatment for ADHD. Generally, teachers with more misperceptions and knowledge gaps regarding ADHD rated children as more impaired on several domains. The results of the current study reveal large gaps in preschool teacher knowledge of ADHD. As such, there are important implications for future teacher training and professional development.