Using the field's state-of-the-art knowledge, we designed, implemented, and assessed the effectiveness of an intervention to help physical education (PE) teachers be more autonomy supportive during instruction. Nineteen secondary-school PE teachers in Seoul were randomly assigned into either an experimental or a delayed-treatment control group, and their 1,158 students self-reported their course-related psychological need satisfaction, autonomous motivation, amotivation, classroom engagement, skill development, future intentions, and academic achievement at the beginning, middle, and end of the semester. Observers' ratings and students' self-reports confirmed that the intervention was successful. Repeated-measures ANCOVAs showed that the students of teachers in the experimental group showed midsemester and end-of-semester improvements in all dependent measures. A multilevel structural equation model mediation analysis showed why the teacher-training program produced improvements in all six student outcomes - namely, teachers in the experimental group vitalized their students' psychological need satisfaction during PE class in ways that teachers in the control group were unable to do, and it was this enhanced need satisfaction that explained the observed improvements in all six outcomes.