Background: Mental health literacy is important for awareness of mental illnesses in adolescence, which is the initial period of mental disorders. Determining the levels of mental health literacy guides studies in activities promoting mental health. This study aimed to determine the mental health literacy levels of high school students and teachers and related factors. Methods: Our study is a cross-sectional study. A total of 1051 people (students and teachers) were included in the Fatih District of Istanbul were included in our study. The mental health literacy levels of the participants were evaluated with the Mental Health Literacy Questionnaire. This questionnaire includes scenarios about mental problems called "vignettes" and questions about them. Results: Students' recognition percentages of depression, schizophrenia, and social phobia vignettes were found to be 28.1%, 46.5%, and 5.9%, respectively. For the person who was described in the vignettes, the "family physician" was chosen as the least by students and teachers. A statistically significant relationship was found between students' mental health literacy levels and sex, grade, father education levels, and experiencing vignette similar issues (p < 0.005). Additionally, teachers who were not school administrators and experienced vignette similar issues levels were higher (p < 0.005). Conclusions: These results showed us that mental health literacy levels are low in high schools. School-based intervention studies on this issue are of critical importance. In addition, it is seen that individuals do not consider asking for help with mental health problems from primary care services. This shows the inadequacy of the integration of primary health care services and mental health services.