The current study aims to determine the associations of insufficient sleep with suicide attempts and self-injury in a large, school-based Korean adolescent sample. A sample of 4553 middle- and high-school students (grades 7-10) was recruited in this study. Finally, 4145 students completed self-report questionnaires including items on sleep duration (weekday/weekend), self-injury, suicide attempts during the past year, the Suicidal Ideation Questionnaire (SIQ), and the Beck Depression Inventory (BDI). A multiple linear regression model showed that higher SIQ scores were associated with longer weekend catch-up sleep duration (p = 0.009), higher BDI score (p < 0.001), and longer time spent in a private educational institute (p = 0.025). The multiple logistic regression analysis revealed that longer weekend catch-up sleep duration (p = 0.011), higher BDI score (p < 0.001), longer time spent in a private educational institute (p = 0.046), and poorer academic record (p = 0.029) were associated with suicide attempt and self-injury during the past year. The present results suggest that weekend catch-up sleep duration - which is an indicator of insufficient weekday sleep - might be associated with suicide attempts and self-injury in Korean adolescents.