Background: The French National Observatory for Physical Activity and Sedentary Behaviors conducted a national survey aiming to evaluate the potential effects of confinement on the population’s physical activity levels and sedentary behaviors. Methods: In close collaboration with the French Ministry of Sports and a selected expert committee, 3 different questionnaires investigating 3 subgroup populations were included in the survey: (1) children, (2) adolescents, and (3) adults. Results: Forty-two percentage of children, 58.7% of adolescents, 36.4% of adults, and 39.2% of older people had reduced physical activity levels. Particularly, active transportation and endurance practices showed a significant decrease, while domestic, muscular strengthening, and flexibility activities increased. Sitting time and screen time increased, respectively, in 36.3% and 62.0% of children, 25.5% and 69.0% in adolescents, 24.6% and 41.0% in adults, and 36.1% and 32.1% in seniors. Conclusion: The COVID-19 confinement period led to important modifications in individual movement behaviors at all ages, particularly favoring decreased physical activity and increased sedentariness. These findings suggest that the authors need to inform and encourage people to maintain and improve their physical activities and to change their sedentary time habits during postconfinement and during the period of a potential future lockdown.