Background: Attention deficit hyperactive disorder (ADHD) is a leading mental disorder among adolescents globally and is associated with premature mortality. Knowing the trends and key determinants of ADHD in youth are critical for earlier diagnosis and interventions. Methods: We retrospectively examined all hospitalizations in patients aged 11 to 18 years with ADHD in Spain, using data from the Spanish National Registry of Hospital Discharges spanning 2000 to 2021. We compared our data with available literature. Results: Over the 22-year study period, there were 2,015,589 hospitalizations among adolescents in Spain, with 118,609 (5.9%) cases involving mental disorders. There were 10,292 admissions with ADHD, representing 8.7% of all hospitalizations among youth with mental disorders. Median age was of 14 years. Boys represented 72.6%. Admissions with ADHD experienced a 17-fold increase during the last decade (p < 0.001). The in-hospital mortality rate for adolescents with ADHD was 0.1%, lower than for other mental disorders. During the first year of the COVID-19 pandemic, admissions decreased but resumed its rising trend thereafter. Conclusion: Hospital admissions for ADHD among adolescents have significantly increased in Spain during the last two decades. This increase outpaced the mild upward trend in ADHD diagnoses over the same period, which is not due to actual increases in the incidence of ADHD, which has remained stable at 5% worldwide. Factors beyond increased awareness, improved identification, and changes in diagnostic criteria must be considered. Boys represented 72.6% of these admissions. In-hospital mortality in adolescents with ADHD was lower than for admissions with other mental disorders.