Based on self-determination theory, this study examines the relations between teachers' perception of classes' engagement, their motivating teaching practices, and students' motivation. Fifty-two physical education (PE) teachers and their 1,040 students from 52 classes participated in this study. Teachers' perception of classes' engagement was self-reported one week before their motivating teaching practices were observed. At the end of the observed lesson, students completed a questionnaire assessing their situational motivation toward PE. Results indicated that teachers' perception of class engagement predicted different dimensions of students' self-determined motivation, and that these relations were fully mediated by teachers' behaviors. More specifically, the more teachers reported high scores in class engagement, the more they displayed need-supportive behaviors that were, in turn, related to positive students' motivational outcomes. These results suggest that teachers' perception of class engagement could exacerbate motivational differences between classes.