Introduction: The health benefits of muscular fitness for youth are compelling. Current guidelines recommend young people (5-17 years) engage in muscle-strengthening activities (e.g., resistance training [RT]) on at least three days per week. However, only 13% of Australians aged 15-17 meet this guideline. Schools present a unique opportunity to introduce adolescents to RT. However, few school-based physical activity interventions have focused on RT, possibly due to reported barriers to delivery in schools. Moreover, the majority of school-based interventions do not progress beyond pilot, efficacy/effectiveness phases to be implemented at-scale. The purpose of this study was to evaluate the state-wide dissemination of the Resistance Training for Teens (RT for Teens) program using the RE-AIM framework. Methods: Student-, teacher- and school-level data related to RE-AIM were collected between August 2015 and October 2020. RE-AIM was operationalised as: (i) Reach: number and characteristics of students estimated to be exposed to the program; (ii) Effectiveness: impact of the program on student-level outcomes measured in a subsample of students (n = 750); (iii) Adoption: number and representativeness of schools with one or more teachers trained to deliver the program; (iv) Implementation: extent to which the program was delivered as intended; and (v) Maintenance: extent to which program delivery was sustained in schools. Results: Estimated program reach was ~10,000 students (~5% of a total student population of ~200,000). Students were from varied socioeconomic and language backgrounds. Program participation improved students’ muscular fitness, RT self-efficacy, perceived cardiorespiratory fitness and flexibility, and participation in muscle-strengthening physical activities. A total of 468 teachers from 249 schools attended program training, with 30 workshops delivered. Schools were located in diverse geographical regions across NSW. Program implementation was measured via lesson observations and we observed considerable variability in the quality of program implementation. Resources were used in the majority of lessons; however, teachers had adapted the program to suit their students and school context. Despite the adaptations that were evident, the necessary focus on RT remained. Adherence to the SAAFE (Supportive, Active, Autonomous, Fair and Enjoyable) teaching principles was high during observed lessons. Fifty-one schools (20.5%) sent an additional (previously untrained) teacher to a second workshop. Discussion: Although the RT for Teens had high levels of reach and adoption, there was considerable variability in the quality of program implementation. Additional research is needed to identify support models to optimise implementation quality and sustain program delivery and effectiveness over time. Trial registration: ACTRN12621000352808 My co-authors and I acknowledge that we have no conflicts of interest of relevance to the submission of this abstract.