Introduction - In the context of the COVID-19 pandemic, as soon as the lockdown was announced—which was in effect from March 17 to May 11, 2020—Santé publique France launched a behavioral surveillance initiative (the CoviPrev survey) to assess the impact of the lockdown on public health. The sixth wave of this survey, conducted from May 4 to 6, 2020, analyzed levels of physical activity and sedentary behavior among the adult population during lockdown, perceived changes in these behaviors compared to pre-lockdown levels, and associated factors. Methods - An independent sample of 2,000 people, aged 18 and older, residing in metropolitan France, was surveyed online. Representativeness was ensured using the quota method, and the data were adjusted according to the 2016 population census. The prevalence of physical inactivity (less than 30 minutes of physical activity per day) and high sedentary behavior (more than 7 hours spent sitting per day), as well as the frequency of breaks from sitting, were analyzed descriptively, along with their associations with sociodemographic and mental health variables. Reported changes in physical activity and time spent sitting compared to before the lockdown were analyzed using multivariate models. Results - During the lockdown period, half of the population did not meet the recommendations of at least 30 minutes of physical activity per day, and one-third reported a high level of sedentary behavior, spending more than 7 hours per day sitting. The frequency of breaks from sedentary behavior was below lockdown-specific recommendations for 55% of respondents. Compared to their pre-lockdown habits, 47% of respondents reported a decrease in physical activity, and 61% reported an increase in daily time spent sitting. The lack of physical activity was more prevalent among individuals in lower socioeconomic categories (CSP-) or those without employment, as well as among women with lower educational attainment, those on sick leave, or those on partial unemployment. In contrast, reporting a decrease in physical activity was associated with higher socioeconomic status and, among men, with teleworking. The reported increase in time spent sitting was associated with teleworking and partial unemployment. Links between physical activity, sedentary behavior, and mental health were demonstrated. Lack of physical activity, high levels of sedentary behavior, a decrease in physical activity, and an increase in time spent sitting compared to before the lockdown were associated, in different ways depending on gender, with anxiety, depression, or sleep disturbances reported during the lockdown. Conclusions - Our results highlight a deterioration in physical activity and sedentary behavior among the adult population during the lockdown period. They demonstrate the importance of promoting an active lifestyle in such a situation to help maintain the population’s physical and mental health. Preventive measures aimed at limiting and breaking sedentary behavior also appear particularly important to continue in the context of telework, which is likely to become more widespread beyond the COVID-19 pandemic.