School teachers are in a unique position to recognize suicide-related problems in their students and to appropriately support them; teachers may need high levels of suicide literacy. However, few studies have examined current levels of suicide literacy in teachers. This study aimed to investigate suicide literacy in school teachers. Teachers (n = 857) from 48 Japanese schools (primary and junior-/senior-high) answered a self-administered questionnaire assessing (a) knowledge about suicide, (b) intention to ask about students' suicidal thoughts/plans, and (c) attitudes towards talking to students with mental health problems. The average proportion of correct answers to the knowledge questions (10 items) was 55.2%. Over half of the teachers knew that suicide is a leading cause of death in adolescents (55.0%), and that asking about suicidality is needed (56.2%). Half of the teachers intended to ask students about their suicidal thoughts (50.2%) and fewer intended to ask about experiences of planning suicide (38.8%). Most of the teachers (90.4%) agreed with the idea that talking to students with mental health problems was a teacher's responsibility. Intention to ask about students' suicidal thoughts/plans were higher in teachers in their 20s (vs. 40s-60s) and working at junior-/senior-high schools (vs. primary schools). Suicide literacy in Japanese school teachers was observed to be limited. However, teachers felt responsibility for helping students with mental health problems. The development and implementation of education programs may help improve teachers' suicide literacy, which, in turn, could encourage effective helping behaviors of teachers for students struggling with suicidality.