 This study examined how the COVID-19 pandemic affected physical activity, PA, patterns, screen time, and sleep among Swedish adolescents. The results showed that moderate to vigorous physical activity, MVPA, remained unchanged while light physical activity, leaper, decreased and sedentary time increased. Additionally, sleep duration decreased and screen time increased. Girls, adolescents with overweight to obesity, BMI and percent body fat, and those with lower cardio respiratory fitness, CVF, at baseline had less favorable changes in PA patterns, sleep and screen time. These findings suggest that certain groups may be more vulnerable to the negative health consequences of the pandemic and require targeted interventions to help them maintain their physical activity levels and reduce sedentary behavior.