Background: Lockdown measures during the first wave of the COVID-19 pandemic in France led to serious public health concerns over people who use illicit drugs, especially in terms of mental health. We assessed changes in cannabis use during the first lockdown in France among daily cannabis users and associated correlates. Methods: CANNAVID is a French, national, cross-sectional web-based survey, conducted from 17 April to 11 May 2020. Daily cannabis users aged ≥ 18 years and living in France were invited to participate through advertisements. Respondents completed an ad hoc questionnaire on a dedicated online platform. We analyzed changes in cannabis use during the first lockdown (i.e., stopped, decreased, unchanged, or increased) and performed a multinomial logistic regression analysis to evaluate correlates of these changes. Results: Of the 4019 participants, 74.0% were men. Median age was 27 years (interquartile range: 22-37). With regard to cannabis use, 293 (7.3%) persons stopped, 1153 (28.7%) decreased, 1146 (28.5%) did not change, and 1427 (35.5%) increased their use during the lockdown. A multinomial logistic regression model revealed several sociodemographic, behavioral and health-related factors associated with changes in cannabis use. Compared with participants with an unchanged level of cannabis use during the lockdown, those who increased and those who stopped cannabis use were more likely to have increased tobacco and alcohol use and to have experienced depression and sleep disorders intensification. Those who stopped cannabis use were also more likely to have increased benzodiazepine use and to have experienced pain increase during lockdown. Conclusions: France's first COVID-19-related lockdown had a differential impact on daily cannabis users' consumption patterns. Most study respondents reported changes to their cannabis consumption pattern. Those who reported a stable cannabis use were more likely to report fewer negative changes. Specific interventions are needed for this population, as well as research to assess the long-term impacts of these changes.