PurposeThe purposes of this study were to assess whether mental well-being has changed after introduction of the lockdown measures compared with that before, whether this change differs between boys and girls, and whether this change is associated with COVID-19–related concerns. MethodsThis is a two-wave prospective study among Dutch adolescents using data collected up to one year before the COVID-19 pandemic (n = 224) and 5–8 weeks after the first introduction of lockdown measures (n = 158). Mental well-being was assessed by three indicators: life satisfaction, internalizing symptoms, and psychosomatic health. General linear model repeated-measures analysis of variance was used to assess whether mental well-being has changed and if this differed by sex. Univariate linear regressions were used to assess associations between COVID-19–related concerns and a change in mental well-being. ResultsLife satisfaction decreased (η2p = .079, p < .001), but no change in internalizing symptoms was observed (η2p = .014, p = .14), and psychosomatic health increased (η2p = .194, p < .001) after the introduction of lockdown measures. Boys scored significantly better on all mental health indicators compared with girls at baseline and follow-up. However, boys' life satisfaction significantly decreased at the follow-up (η2p = .038, p = .015), whereas girls' life satisfaction did not change. Concerns about COVID-19 were significantly associated with a lower life satisfaction and more internalizing symptoms. ConclusionsAdolescents', especially boys', life satisfaction decreased during the lockdown. They reported no change in internalizing symptoms and an improved psychosomatic health. Adolescents' mental well-being is expected to vary during the COVID-19 pandemic and should continue to be monitored.