The current study examined the relationships between in-service and pre-service school psychologists’ knowledge of ADHD, ADHD stigma perceptions, and level of education and experience. Forty-five in-service school psychologists and 27 pre-service school psychologists completed online self-report measures. Results indicate that in-service school psychologists have significantly higher ADHD knowledge than pre-service; however, these groups did not differ in their ADHD stigma perceptions. Results also suggest that it is only ADHD-specific experience, and not knowledge of ADHD, years of experience, or education, that relate to ADHD stigma perceptions. These findings have implications for the training of school psychologists, as well as possible interventions for improving knowledge and perceptions of stigma in ADHD.