BACKGROUND: High levels of mental health problems among young people were reported during the COVID-19 pandemic, but studies of the post-pandemic period are scarce. We assessed mental health problems among Finnish youth before, during, and after the COVID-19 pandemic using nationwide population-based samples. Our aim was to examine in which direction the heightened levels of adolescent mental health problems have developed after the pandemic. METHODS: In this national, repeated cross-sectional, population-based study in Finland, we recruited students at lower and upper secondary level (aged 13-20 years) who were taking part in the Finnish School Health Promotion (SHP) survey in 2015-23 (119 681-158 897 participants per round). The SHP is based on total sampling and conducted biennially between March and May. Self-reports covered the seven-item Generalized Anxiety Disorder Scale; the two-item Patient Health Questionnaire for depression; the Mini Social Phobia Inventory for social anxiety; the Short Warwick-Edinburgh Mental Wellbeing Scale for mental wellbeing; loneliness; the Sick, Control, One Stone, Fat, Food measure for disordered eating; and suicidality (suicidal ideation, deliberate self-harm, and suicide attempts). Scales were dichotomised using validated cutoffs. Presence of any and comorbid mental health problems was assessed. Logistic (for dichotomised outcomes) and linear (for Short Warwick-Edinburgh Mental Wellbeing Scale) mixed effects models were used to analyse the effect of survey year on mental health, controlling for sociodemographic background factors and stratified by gender and school level. Cisgender and transgender youth were compared. FINDINGS: Between 2015 and 2023, the SHP study recruited 722 488 students (371 634 [51·6%] girls and 348 857 [48·4%] boys) with a mean age of 15·8 years (SD 1·3) who were either in the eighth and ninth grades of comprehensive school or the first and second years of general and vocational upper secondary schools in Finland. The proportion of participants with generalised anxiety, depression, and social anxiety symptoms above the cutoff increased from pre-COVID-19 levels to 2021 and remained at these higher levels in 2023 among all study groups. Among girls in lower secondary education, prevalence of generalised anxiety, depression, and social anxiety symptoms increased from 2021 to 2023, as did social anxiety among girls in upper secondary education. Among boys, the proportion with social anxiety symptoms decreased between 2021 and 2023. Mental wellbeing scores decreased in all groups between 2021 and 2023, and disordered eating increased in girls, and in boys in lower secondary education. Suicidality increased in girls but not in boys. Loneliness was the only measure to show improvement in all groups from 2021 to 2023. In 2023, 55 895 (72·6%) of 76 994 girls and 22 718 (32·8%) of 69 205 boys reported at least one mental health problem, and 37 250 (48·4%) girls and 9442 (13·6%) boys reported comorbid mental health problems. Among both transfeminine and transmasculine youth, the prevalence of generalised anxiety and depression symptoms decreased from 2021 to 2023, but compared with cisgender youth, the proportions were significantly higher throughout. INTERPRETATION: The effects of the COVID-19 pandemic on youth mental health could be long lasting. In this study, the substantial change for the better among transgender youth was a positive exception. Providing adequate support and treatment for young people with poor mental health is essential, but solutions to the mental health crisis need to address a wider societal perspective and should be developed in partnership with young people. FUNDING: NordForsk, Research Council of Finland. TRANSLATIONS: For the Finnish and Swedish translations of the abstract see Supplementary Materials section.