Introduction: Secondary schools have the potential to promote health-related fitness (HRF) and physical activity within and outside school hours. As such, schools are often chosen as the setting to implement child and adolescent physical activity programs. School-based programs often utilise teachers as delivery agents, but few studies examine effects on teacher-level outcomes. Purpose: The primary aim of this study was to determine the impact of teacher training embedded within a physical activity intervention on teacher-level outcomes. The secondary aim of this study was to evaluate process data, including implementation, satisfaction and fidelity. Methods: "Resistance Training for Teens" ("RT for Teens") was evaluated using a cluster randomised controlled trial in 16 secondary schools. Teachers (N = 44; 48% female/52% male; mean ± SD years teaching experience = 10.6 ± 8.0) from 16 secondary schools were assessed at baseline. Intervention group teachers (i.e. from eight schools) delivered a structured school-based physical activity program over 10-weeks. Teacher outcomes included confidence to teach health-related fitness (HRF) activities, perceived barriers to teaching HRF activities, and perceived fitness. Detailed process evaluation data were also collected. Assessments were conducted at baseline and 6-months (post-program), and outcomes were assessed using repeated measures analysis of variance. Results: There was a positive group-by-time effect for the confidence composite score (p = 0.010, partial eta squared = 0.29), but no effects for the two (contextual, interpersonal) barrier composite scores. Also, there was a significant effect for perceived 'general fitness' (p = 0.044, partial eta squared = 0.13), but not for specific fitness subdomains. Teachers were highly satisfied with both the training and the program, believing it was beneficial for students. Resource usage and adherence to the SAAFE (Supportive, Active, Autonomous, Fair, Enjoyable) delivery principles was high. Conclusion: "RT for Teens" improved teachers' confidence and perceived fitness. These findings highlight the potential for high-quality teacher training and program delivery to positively influence teacher-level outcomes. This may provide support for the use of teacher professional development to improve HRF-related pedagogy.