Introduction - To curb the COVID-19 pandemic, a strict lockdown was implemented in France from March 17 to May 11, 2020, drastically disrupting people’s daily lives. From the start of the lockdown, a repeated survey by Santé publique France (the CoviPrev survey) tracked various health behaviors. This article presents the perceived changes in dietary behaviors during the lockdown and the determinants of weight gain. Methods - Two independent samples of 2,000 people aged 18 and older residing in metropolitan France were surveyed online during the third (V3, 5th week of lockdown) and sixth (V6, 8th week of lockdown) survey waves. In V3, changes in eating behavior, physical activity, and weight were analyzed descriptively, and logistic regressions assessed the factors associated with weight gain. Repeating the survey in V6 allowed us to describe changes in snacking and weight between the two waves, comparing the data using chi-square tests. Results - In V3, 37% of individuals reported having “changed their diet,” with no difference by gender. However, more women than men reported cooking more often, snacking more, consuming fatty, sugary, and salty foods (PGSS), having more or less appetite, and paying attention to their weight. Among the behaviors assessed, the majority were impacted by the lockdown for 25 to 40% of respondents: for example, 37% reported cooking homemade meals more frequently than usual, and 27% reported gaining weight compared to 11% who lost weight. Positive associations were observed between weight gain and a financial situation perceived as difficult, eating larger quantities than usual, snacking more, reducing fruit and vegetable consumption, increasing FSS consumption, engaging in less physical activity, experiencing definite depression, and having sleep disorders. Finally, 27% of respondents reported snacking more in V6 compared to 22% in V3, and 36% reported gaining weight compared to 27% in V3. Conclusions - This study demonstrates the impact of lockdown on several dietary behaviors, particularly among women. While it may have encouraged home cooking, other changes were generally detrimental to health. These results suggest that repeated lockdowns could exacerbate diet-related health conditions. In such contexts, it is necessary to continue monitoring eating behaviors and body weight and to support all public health measures that encourage healthy eating habits.