Aim: A worry regarding young people’s physical activity engagement relates to a potentially competing development: the role of screen-based media (SBM) in their everyday lives. The present study aimed to assess time spent on different types of SBM, self-reported moderate-to-vigorous physical activity (MVPA) and physical activity in different domains and their interrelations. Methods: The study is based on data from 3737 participants aged 11, 13 and 15 years from Norway collected in 2021/2022 as part of the Health Behaviour in School-aged Children collaborative cross-national survey. Results: The findings revealed high amounts of total SBM time. There were age and gender differences in physical activity, especially with lower levels of active transport during leisure time among girls than among boys. A series of binary logistic regression analyses with adjustment for age, gender and socioeconomic status showed that high levels of total SBM time were negatively associated with involvement in club sports, active school transport, active travel to friends and leisure activities, and 60 min MVPA 5 days/week. Gaming and social media use were also negatively associated with most of the physical activity indicators. All SBM variables were negatively associated with involvement in club sports and 60 min MVPA 5 days/week. Conclusions: The study demonstrates negative associations between SBM time among adolescents and physical activity in various domains. Actions to facilitate youth physical activity in the digital age seem crucial. Additional studies with nuanced data on these behaviours and longitudinal research design allowing for examining their interrelations over time are needed.