Background Plain packaging (PP) of tobacco products and increased graphic warnings may contribute to lower attractiveness of smoking, particularly among youths. In France, this policy was introduced on 1 January 2017. We examined changes in smoking-related perceptions and behaviours among a nationwide sample of French adolescents before (2016) and 1 year post (2017) implementation. Methods DePICT is a two-wave cross-sectional national telephone survey of adolescents aged 12–17 years per study wave (2016: n=2046 2017: n=1999). All participants reported smoking-related perceptions, as well as ever and current tobacco use. Smokers were also asked about their perceptions of tobacco brands. Data were weighted to be representative of youths in the French population: adjusted prevalence ratios (PRs, 95% CI) estimating changes between the two study waves were calculated using multivariate log-binomial regression models. Results In 2017, as compared with 2016, French adolescents were more likely to report fear of the consequences of smoking (PR=1.06, 95% CI 1.02 to 1.09) and that smoking is dangerous (PR=1.08, 95% CI 1.05 to 1.11). They were also less likely to report that their friends (PR=0.61, 95% CI 0.54 to 0.70) and family (PR=0.51, 95% CI 0.44 to 0.60) accept smoking. Additionally, smoking initiation significantly decreased (PR=0.96, 95% CI 0.93 to 0.98) and a non-statistically significant drop in current tobacco use was observed (PR=0.93, 95% CI 0.78 to 1.11). Smokers’ attachment to their tobacco brand also decreased (PR=0.47, 95% CI 0.30 to 0.73). Conclusion Our findings suggest that PP and increased graphic warnings could contribute to changes in smoking norms and rates among adolescents.