Research has examined the detrimental effects of the coronavirus disease (COVID-19) pandemic on health and physical fitness in adolescents; however, studies comparing these parameters before and after the COVID-19 outbreak have been scarce. Therefore, this study investigated differences in perceived health status, perceived physical fitness, and participation in physical activity among adolescents in the Republic of Korea before and after the COVID-19 outbreak. We chose a sample of data from 2102 adolescents aged 14–19, collected as part of a national survey by the Republic of Korea Ministry of Culture, Sports, and Tourism from 2019 to 2021. We focused on five items from the survey related to health awareness and physical activity. Although perceived health status was lower in 2021 than in 2019 or 2020, there were no differences in perceived physical fitness during the three years of the study. Regular participation in physical activity was less common in 2020 than in 2019 or 2021. The proportion of adolescents reporting sufficient rest and sleep was lower in 2021 than in 2020. In addition, fewer adolescents reported eating regular meals and engaging in nutritional supplementation in 2021 than in 2019 and 2020. Rates of abstinence from alcohol and smoking cessation were higher in 2021 than in 2019 or 2020. For all three years, adolescents reported the following as the primary reasons for engaging in regular physical activity (in order): “maintenance of mental health”, “maintenance of physical health”, “help in daily life”, and “reduction of medical expenses”. In preparation for the post-COVID-19 era, these results highlight the need to prepare measures and countermeasures to promote health and physical activity among adolescents in the Republic of Korea.