Background In view of experts’ warnings about the potential negative mental health consequences of the sudden nationwide lockdowns implemented in many countries to limit the spread of the COVID-19 pandemic, we sought to study the incidence of posttraumatic stress disorder (PTSD) after traumatic events related to this unprecedented lockdown in the French general population. Methods This longitudinal study among adults (aged =18) consisted of two surveys: the first during the last days of the lockdown and the second a month later. We estimated PTSD incidence with the PCL-5 and ran multiple Poisson regression models to identify factors associated with PTSD. Results: Among the 1736 participants, 30.1% reported at least one traumatic event. PTSD incidence was 17.5% (95% confidence interval CI = 15.7–19.3). It was higher in participants who reported multiple traumatic events, who had high COVID-19-related media use, who had general anxiety disorder (GAD-7) during the lockdown, and who had GAD, depression (PHQ-9), or sleep problems 1 month later. In addition, 43.1% of people with PTSD reported suicidal thoughts. Conclusions These results should help clinicians to target people who are at high risk of developing PTSD after a pandemic-related lockdown and could benefit from preventive measures. Collaboration between the media and mental health professionals could be envisioned to inform the population about care resources. Follow-up recommendations should also be disseminated to general practitioners to facilitate PTSD screening and ensure that they are aware of the appropriate management.