Background: Despite the proven health benefits of physical activity (PA), more than a quarter of the world’s adult population is insufficiently physically active, which increases the risk of diseases and premature death. During the COVID-19 pandemic governments have implemented enforced measures to limit the spread of the virus, which could affect people’s PA. Aim: This study aimed to examine changes in PA levels, types of PA, and sedentary behavior in the Swedish population before and during the COVID-19 pandemic. Associations between demographical and behavioral determinants, and changes in PA were also investigated. Method: In December 2020, 1.035 individuals (18-79 y) completed a survey about PA and sedentary behavior, at present and before the pandemic. Factors influencing their PA were also explored. Results: Fifty-one percent of the sample reduced total PA, 18% had no change and 31% increased PA. Overall, organized PA, for example at fitness centers and sport training, decreased the most, and sedentary behavior increased. The youngest and the oldest age groups reported the greatest reduction of PA, while the middle age groups reported the most increased PA. Mental and physical capability was associated with PA change. Conclusion: During the pandemic, most of the Swedish population have decreased PA levels with a concurrent increase in sedentary behavior. This may have both a short-and long-term negative health impact on individuals’ and populations’ health. Implications for caring in a changing world: As physical inactivity already is a global public health problem during non-pandemic circumstances this study’s findings call for further actions during and after the pandemic to support people in maintaining or increasing daily PA and decreasing sedentary behavior. Interventions in health care are recommended to strengthen peoples’ ability to perform PA, focus on the youngest and oldest age groups, and be applied on an individual basis as well as in organizational contexts.