To plan effective school-based adolescent suicide prevention strategies, there is need for valid epidemiology data. The aim of this study was to estimate the current (1�month) prevalence and associated factors of suicidal behaviours (ideation, planning, and attempt) amongst secondary school adolescents in Lagos, Nigeria. A total of 9441 adolescents (4684 males and 4757 females) with mean age 15.61�years (SD 1.49) recruited from 47 public senior secondary schools self-completed questionnaire consisting of sociodemographic, family, school, physical health, and mental health-related variables. Suicidal behaviours (ideation, planning, and attempt) in the past 1�month were also assessed. The weighted 1-month prevalence for suicidal ideation was 6.1% (95% CI 5.5–6.7), suicidal planning was 4.4% (95% CI 3.8–4.9), and suicidal attempt was 2.8% (95% CI 2.4–3.2). The factors significantly associated with suicidal behaviours included being female, not staying with the mother, maternal drinking, witnessing domestic violence, past and present academic difficulties, having no close friend in school and having problems relating with peers and teachers. Also, presence of chronic physical illnesses, depression, anxiety, behavioural disorders, and psychotic-like experiences were associated with adolescent suicidal behaviour. We have shown that a substantial percentage of adolescent have suicidal behaviours, and that there were demographic, family, school, physical health, and psychological health-related factors. We believe that our findings will be important when planning suicide prevention services that could be incorporated into the school mental health services