Background: teen Mental Health First Aid (tMHFA) is an Australian school-based universal program for grade 10 to 12 students. tMHFA teaches teens how to recognize and respond to a peer in crisis or experiencing mental health concerns. Methods: Schools implementing tMHFA in 2019 and 2020 were propensity score matched, yielding a sample of instructors (n = 130) and students (n = 1915) in 44 high schools in 24 American states. Effectiveness and acceptability were assessed with student surveys at baseline and after implementation. Results: There were significant findings for primary outcomes, including improved helpful first aid intentions (Cohen ds = 0.57 to 0.58), confidence supporting a peer (ds = 0.19 to 0.31); the number of adults rated as helpful (ds = 0.37 to 0.44); and reductions in stigmatizing beliefs (ds = 0.21 to 0.40) and "harmful first aid intentions" (ds = 0.11 to 0.42). Instructors and students rated the program favorably with students sharing improvements on their recognition and responses to mental health problems and crises. Conclusion: tMHFA is an effective, feasible, and scalable training program for increasing mental health literacy and decreasing mental health stigma in adolescents in the short term, consistent with trials of tMHFA in Australian adolescents.