Background: The coronavirus disease 2019 (COVID-19) pandemic has negatively impacted many aspects of life. In particular, measures for combatting the spread of the virus, such as quarantining and restrictions on social gatherings, may have led to psychological anxiety and depressive symptoms among adolescents. Such mental health impacts may increase the risk of suicidal ideation. This study aimed to examine mental health among adolescents amid the prolonged COVID-19 pandemic, and identify and analyze predictors of suicidal ideation, suicide planning, and suicide attempts.Methods: Data for 57,925 adolescents who participated in the 2020 Korea Youth Risk Behavior Web-based Survey were used for this research. Based on their responses to suicide-related questions, the sample was divided into a healthy group, suicide-ideation group, suicide-planning group, and suicide-attempt group, respectively. The groups’ descriptive statistics were then analyzed. An analysis of covariance, post-hoc tests, and multiple logistic regression were performed on the four groups.Results: Overall, 6.9% of the participants reported suicidal ideation, 2.2% reported planning suicide, and 1.9% reported attempting suicide in the previous 12 months. Stress, sadness and despair, loneliness, and generalized anxiety disorder were all highest in the suicide-attempt group and lowest in the healthy group. Sex, academic achievement, economic status, impact of COVID-19 on economic status, sleep, alcohol use, drug use, smartphone overdependence, and suicide-associated factors significantly predicted adolescents’ mental health status. Conclusions: Amid the prolonged COVID-19 pandemic, there is a strong need for various individualized programs that identify and intervene to support adolescents with a suicidal risk by accurately assessing their mental health risk factors, such as stress, sadness and despair, loneliness, and generalized anxiety disorder.