Mental health literacy (MHL) is an essential component in the process of de-stigmatization, promoting mental health, and supporting people struggling with mental illness. Today, as the number of people suffering from mental illnesses is nearly 450 million people worldwide, the importance of having a teacher-training program that incorporates MHL in its curriculum has become paramount. This study is a quasi-experimental pre–post research that uses the MHL program as an intervention. The sample included 36 (n = 36) high school in-service teachers from 11 schools. The participants took an online training program for 6 weeks. The evaluations were used before and after the training to assess their mental health knowledge, attitude, and self-efficacy. The data were analyzed using the paired sample t-test. The findings revealed a significant level of improvement in relation to the teachers’ MHL knowledge, attitude, and self-efficacy after the training. The study findings reveal the effectiveness of MHL training in improving in-service teachers’ knowledge, attitude, and self-efficacy in relation to mental health. The study’s limitations and future study recommendations are discussed.