This study examined the reliability and validity of the Classroom Assessment Scoring System–Secondary (CLASS-S) in Finnish classrooms. Trained observers coded classroom interactions based on video recordings of 46 Grade 6 classrooms (450 cycles). Concurrent associations were investigated with respect to teacher self-ratings (e.g., efficacy beliefs and teaching-related stress). Confirmatory factor analysis showed that the hypothesized three-factor structure of the original CLASS-S (Emotional Support, Organizational Support, and Instructional Support), with some modifications, provided a better fit for the data compared with one- and two-factor structures. Structural validity was demonstrated by mostly high factor loadings. Except for two interrater intraclass correlations, all item, scale, and interrater reliabilities were either acceptable or good. The study found some evidence for concurrent associations between the three CLASS-S factors and teacher self-ratings. The results provide evidence of the applicability of the CLASS-S instrument in educational contexts (Finland) outside the United States.