Due to the COVID-19-induced social distancing restrictions, adolescents’ physical activity declined and their level of health and fitness decreased. In March 2023, the Korean government established the beginning of the post-COVID-19 era by declaring that indoor masks were now “recommended” rather than “mandatory”. Consequently, adolescents, whose physical activity decreased during COVID-19, began to participate in such activities again. This study aimed to verify the differences in adolescent physical activity during COVID-19 and after COVID-19. To achieve the study’s purpose, an online survey was conducted twice, using the International Physical Activity Questionnaire, for 1143 Korean adolescents in 2022 and 2023. The following results were derived through frequency analysis, descriptive statistical analysis, and an independent variablest-test. First, moderate-to-vigorous physical activity was higher during the post-COVID-19 period than during COVID-19 (p= 0.018). Second, high-intensity (p= 0.018), moderate-intensity (p= 0.030), and low-intensity (p= 0.002) physical activities and total leisure-time physical activities (p= 0.003) were all higher during the post-COVID-19 period than during COVID-19. Third, high-intensity (p= 0.005), moderate-intensity (p= 0.003), low-intensity (p= 0.003) activities and total physical activities in school (p= 0.001) were all higher during the post-COVID-19 period than during COVID-19. Fourth, there was no difference in the commuting times for cycling (p= 0.515) and walking (p= 0.484) and the total physical activities during commuting (p= 0.375) during and after COVID-19. Based on these results, the methods to help adolescents form correct habits for leading a healthy life are discussed.