There is limited post-pandemic youth mental health data in low- and middle-income countries. This study describes the prevalence of suicidal ideation, suicidal attempt, and self-harm since the COVID-19 pandemic among young Filipino adolescents. Adolescents aged 13-16 years old from public and private high schools in Cavite, Philippines were recruited for a cross-sectional school survey conducted from May 2023 to February 2024. Suicidal behaviours and self-harm since the pandemic were determined using a self-administered questionnaire alongside sociodemographics and internalising and externalising symptoms. Of the 1,229 13-16-year-olds who completed the survey, 54.0% experienced suicidal ideation, 24.2% attempted suicide, and 34.2 % reported self-harm between 30 January 2020 and the date when they completed the survey. The prevalence of suicide attempts was higher among females (29.6%) than males (13.1%). Parental absence was associated with suicidal attempts (ARRR=2.93) and self-harm and/or suicidal ideation (ARRR=2.00) while living with either the biological mother or father was moderated by gender. Internalising and externalising symptom scores increased the risk for both outcomes by ≥15%. This study revealed a high prevalence of suicidal and self-harming behaviours among young adolescents in the Philippines. This calls for action to implement population-based strategies in suicide prevention, early screening, and cross-sectoral intervention.