Key PointsQuestionHow has the long-term trend of suicidality among Korean youths changed during the COVID-19 pandemic?FindingsIn this nationwide serial cross-sectional survey study from 2005 to 2021 of 1 109 776 Korean adolescents aged 13 to 18 years, the slope of the long-term trends in suicidality decreased in the prepandemic period (from 23.0% in 2005-2007 to 12.3% in 2016-2019), whereas the slope increased during the COVID-19 pandemic (from 10.7% in 2020 to 12.5% in 2021). The risk factors for suicidality during the pandemic were younger age, female sex, urban residence, smoking, current alcohol use, and low economic status.MeaningThese results can help improve the understanding of suicidality during the COVID-19 pandemic.