Background: Internet addiction (IA) is one of the most prevalent mental health problems among adolescents. Several studies have reported that the prevalence of IA among adolescents has increased during the COVID-19 pandemic. However, with China’s ongoing regular epidemic prevention and control, it is unclear whether the prevalence among adolescents is still at such high levels in China during the COVID-19 pandemic. Therefore, we conducted a cross-sectional study from December 2020 to January 2021 that aims to understand the prevalence and related factors of IA among adolescents in Nanchang, China, during the pandemic. Methodology: The cross-sectional survey was conducted in seven secondary schools in Nanchang. Participants completed a questionnaire comprising Young’s Diagnostic Questionnaire for internet addiction (YDQ), International Physical Activity Questionnaire Short Form, and questions regarding demographic characteristics, outdoor activity time, and sleep quality. Results: A total of 6,553 secondary school students participated in the survey. Of the total, 14.4% (946/6553) were classified as addicted internet users (YDQ ≥5). The risk of IA was higher among students of older age (OR=1.17) and with poorer subjective sleep quality (OR=1.27-2.56). Urban household registration (OR=0.80), high level of physical activity (OR=0.68), 1-2 hours of outdoor activity per day (OR=0.69), long sleep duration (OR = 0.60), and normal sleep duration (OR = 0.74) were protective factors of IA. Conclusions: During the COVID-19 pandemic, adolescents in China reported some risks for IA. Age, household registration, outdoor activity time, level of physical activity, subjective sleep quality, and sleep duration were the key factors affecting IA. Therefore, extended school and family support should be provided for vulnerable adolescents in this extraordinary period.