Objective: To assess attention-deficit/hyperactivity disorder (ADHD) symptoms in the classroom, most often teacher rating scales are used. However, clinical interviews and observations are recommended as gold standard assessment. This systematic review and meta-analysis evaluates the validity of teacher rating scales. Method: Twenty-two studies ( N = 3,947 children) assessing ADHD symptoms using teacher rating scale and either semi-structured clinical interview or structured classroom observation were meta-analyzed. Results: Results showed convergent validity for rating scale scores, with the strongest correlations ( r = .55–.64) for validation against interviews, and for hyperactive–impulsive behavior. Divergent validity was confirmed for teacher ratings validated against interviews, whereas validated against observations this was confirmed for inattention only. Conclusion: Teacher rating scales appear a valid and time-efficient measure to assess classroom ADHD; although validated against semi-structured clinical interviews, there were only a few studies available. Low correlations between ratings and structured observations of inattention suggest that observations could add information above rating scales.