Background Secondary schools are increasingly supporting adolescents’ mental health and well-being, yet many teachers report that they lack the skills and confidence to do so. Building Educators’ skills in Adolescent Mental Health (BEAM) is a web-based training program developed to improve secondary school teachers’ knowledge and confidence in caring for students’ mental health. Objective This pilot study examined the preliminary effectiveness and acceptability of the BEAM program for improving mental health knowledge, attitudes, confidence, helping behaviors, and psychological distress among secondary school teachers. Methods A single-arm pilot trial was conducted from July to December 2019 among secondary school teachers located in New South Wales, Australia, who were employed in leadership positions responsible for managing student well-being (ie, Year Advisors). Participants had access to the BEAM program for 6 weeks. Self-report surveys, delivered at baseline, postintervention (6-weeks post baseline) and 3-month follow-up (19 weeks post baseline) were used to measure changes in training outcomes. Acceptability was assessed by program use, barriers, satisfaction, and participants’ perceptions of program effectiveness. Results A total of 70 secondary school teachers took part (mean age 36.5 years, SD 9.41 years, range 24-60 years). Significant improvements in confidence were reported at postintervention and 3-month follow-up. Significant improvements in helping behaviors were reported at 3-month follow-up only. There was also a significant reduction in psychological distress at postintervention. Participants agreed that the program content was easy to understand and relevant, but program completion was challenged by lack of time, competing priorities, and forgetfulness. Conclusions Findings indicated that a web-based training program may be beneficial for improving secondary school teachers’ abilities to care for students’ mental health; however, program modifications are required to increase training completions. Trial Registration Australian New Zealand Clinical Trials Registry (ANZCTR) ACTRN12619000821190, Universal Trial Number U1111-1232-7680; https://www.anzctr.org.au/Trial/Registration/TrialReview.aspx?id=377529