Background Psychiatric diagnoses and psychotropic medication use among children, adolescents, and young adults have increased in high-income countries, with concerns about geographic variation. We described national trends and geographic variation in Denmark from 2010 to 2025 and developed an interactive online tool to explore these patterns. Methods We conducted a nationwide, descriptive, repeated cross-sectional register-based study of all Danish residents aged 0-24 years in 2010-2025 (n=2,930,744), using the Danish National Patient Registry and the Danish National Prescription Registry. We estimated annual incidence, prevalence, and, for selected diagnoses, lifetime prevalence, by age and sex, and age- and sex-standardised rates across regions, health councils, and municipalities. We assessed geographic variation using deviations from national means, coefficients of variation, Gini coefficients, and funnel plots. Findings Psychiatric diagnoses and psychotropic medication use increased from 2010 to 2025. The annual prevalence of attention-deficit hyperactivity disorder (ADHD) increased from 0.41 to 2.3 per 100 individuals, autism spectrum disorders from 0.20 to 1.2, and melatonin use from 0.14 to 3.0. ADHD and autism spectrum disorders were more prevalent among boys and anxiety disorders among girls. Sertraline use was also more prevalent among girls. Geographic variation was observed across diagnoses and psychotropic medications at regional, health council, and municipal levels. For example, lifetime prevalence of autism ranged from 1.3 to 5.9 per 100 across municipalities, and in 2025 regional coefficients of variation ranged from 0.11 (sertraline) to 0.41 (atomoxetine). Results can be explored in the interactive online tool (https://bupstat.sdu.dk/?lang=en). Interpretation Psychiatric diagnoses and psychotropic medication use increased among Danish children, adolescents, and young adults, with marked geographic variation. Descriptive data cannot establish the causes of this variation, but the findings identify areas for further review and support continued monitoring. Funding Best for Us project (www.bedstforos.dk), funded by the Novo Nordisk Foundation (NNF24SA0093635) and the Obel Family Foundation (69348).