This study investigated the short-term (in 2020) and long-term (in 2021) impacts of the COVID-19 pandemic on suicide-related characteristics in Korean adolescents in comparison with the pre-pandemic period (in 2019) and examined the factors associated with those impacts. Secondary data of the cross-sectional 15th–17th (2019–2021) Korea Youth Risk Behavior Web-based Survey targeting adolescents in school were utilized. The proportions of adolescents with depression, suicidal ideation, suicide planning, and suicide attempts were 26.5%, 12.2%, 3.7%, and 2.4%, respectively. Following an adjustment, depression, suicidal ideation, and suicide attempts significantly improved in the short term and depression and suicide attempts improved significantly in the long term (i.e., 2021), albeit to a lesser degree. The associated variables in the short-term analysis (i.e., 2020) reflected the socioeconomic vulnerabilities (e.g., lower household socioeconomic status [SES], unhealthy status, and unhealthy behaviors), as well as the socioeconomically favorable indicators (e.g., high household SES and high academic achievement). In the long-term, suicidal ideation was no longer associated with a lack of engagement in hand washing, and suicide attempts were no longer associated with the amount of internet time used for studying. For a successful suicide prevention, it is necessary to develop in-school interventions that address the relevant factors identified in this study and the community-based interventions that target out-of-school adolescents.