Few studies have investigated the contagion effects of exposure to others’ confided suicidal thoughts among adolescents. The aim of this study was to investigate whether recent real-world exposure to confided suicidal thoughts was associated with adolescent self-harm and suicidality within one month, and if so, to determine the related risk and protective factors. A total of 5,879 Taiwanese first-year senior high school students participated in this study and completed on-line questionnaires regarding their experience of exposure to confided suicidal thoughts, self-harm and suicidality, socio-demographic information, Patient Health Questionnaire 9-item, Multi-Dimensional Support Scale, Rosenberg Self-Esteem Scale, and alcohol and cigarette use. Of the enrolled adolescents, 3.4% had been exposed to confided suicidal thoughts (N = 200) within one month. The most common source of exposure was friends (N = 186). After controlling for a large variety of confounders, the adolescents with recent exposure to others’ suicidal thoughts were four times more likely to harm themselves within one month (OR = 3.77; 95% CI: 2.26 – 6.30). The risks of suicidal ideation and suicide plans were also markedly increased (both OR = 5.15; 95% CI: 3.48 – 7.64 and 2.78 – 9.52, respectively). The risk ratios were highest if the source of exposure was from parents, and the contagion effects were dose-dependent. Support from parents and teachers could protect the adolescents from the impact of contagion. Our results suggest a contagion effect of exposure to confided suicidal thoughts among adolescents. Parents and school personnel should be aware of this and provide assistance for those in need.