Objectives This study aimed to investigate the changes in the mental health of adolescents after the coronavirus disease 2019 (COVID-19) pandemic. Methods The data for the study were obtained from the Korea Youth Risk Behavior Web-based Survey (KYRBWS), 2020–2021. Overall, the data of 88184 students were included. The data were analyzed using multiple logistic regression analysis with complex sampling using weighted values. The odds ratios (ORs) with 95% confidence intervals (CIs) for subjective stress perception, loneliness, pan-anxiety disorder scale, suicide-related indicators, and high-risk smartphone usage groups were used as major variables. Results The results suggest that the mental health of adolescents deteriorated in 2021 compared to 2020. Adolescents experiencing stress (OR=1.18; 95% CI, 1.08–1.16; p<0.01) and reporting loneliness were also higher (OR=1.07; 95% CI, 1.02–1.12; p<0.01). The high-risk group for pananxiety disorders also increased more than in 2020 (OR=1.05; 95% CI, 1.00–1.11; p<0.01). It was also found that adolescents experiencing suicide accidents increased (OR=1.09; 95% CI, 1.04–1.16; p<0.01). In addition, the high-risk group for smartphone use also increased compared to 2020 (OR=1.17; 95% CI, 1.12–1.23; p<0.01). On the other hand, there was no statistically significant difference in depression, suicide plans, and suicide attempts. Conclusion This study found that the mental health problem of adolescents, which had temporarily decreased after the COVID-19 pandemic, was deteriorating again. These findings suggest that it is necessary to follow up and closely monitor the mental health problems of adolescents from the longitudinal perspective during the COVID-19 pandemic.