Background: Recent literature points to a decline over time in adolescent mental well-being but results are inconsistent and rely mainly on data from Western societies. This study investigates time trends in adolescent mental well-being (psychological and somatic complaints, life satisfaction) among Czech adolescents and explores whether these time trends are moderated by gender, age and socio-economic status. Methods: Nationally representative data from 29,378 Czech adolescents (50.8% girls, M age = 13.43; SD age = 1.65) across five Health Behaviour in School-aged Children (HBSC) surveys (2002, 2006, 2010, 2014, and 2018) were included in the analyses. Hierarchical regression models estimated national trends in adolescent mental well-being. We also tested whether these trends vary between girls and boys, adolescents of different age and from different socio-economic backgrounds. Results: Across the quadrennial surveys from 2002 to 2018, an increase in the psychological complaints was observed. Life satisfaction decreased over time up to 2014 only, whereas somatic symptoms increased until 2010, followed by a decline in 2014 and 2018. Girls, older adolescents and those from low family affluence reported poorer mental well-being outcomes. Gender gap increased over time for psychological complaints and life satisfaction. Socio-economic inequalities gap in adolescent mental well-being remained stable over the investigated timeframe. Conclusions: Our findings do not provide evidence for substantial temporal changes in mental well-being among adolescents in Czechia. Yet, only the increase in psychological complaints has been consistent which is an indicator of a decline over time in adolescent mental well-being. Furthermore, the gender gap in reporting psychological complaints and life satisfaction increased over time, whereas the age and socio-economic differences remained relatively stable. This calls for the attention of public health professionals and policy makers from Czechia. Keywords: Adolescence; mental health; mental well-being; well-being; gender; trends; age; socio-economic status; HBSC