Background: Studies on the prevalence of Atopic Dermatitis (AD) for the adolescent cohort in general-based large populations are scarce worldwide. We performed a retrospective population-based observational cohort study of 76,665 adolescent patients diagnosed with AD in Catalonia (Spain). We study the prevalence of AD by age, gender, disease severity, comorbidities, and serum total Immunoglobin E (tIgE) and appropriate medical treatment (AMT) for the Catalan population. Methods: Adolescent individuals (12-17y) diagnosed with AD by medical records at different health care levels (primary, hospital, emergency) from the Catalan Health System (CHS) were included. Statistical analyses evaluated sociodemographic characteristics, prevalence, comorbidities, serum tIgE and AMT. Results: The overall diagnosed AD prevalence in the adolescent Catalan population (76,665) was 16.9%, being higher for the non-severe (16.7%) than for the severe (0.2%) populations. Topical corticosteroids were the most prescribed drug (49.5%), and the use of all prescribed treatments was higher in severe AD patients, especially systemic corticosteroids (49.7%) and immunosuppressants (45.4%). AD patients had, on average, a serum tIgE of 163.6 KU/L, which was higher for the severe than the non-severe disease (155.5 KU/L vs 101.9 KU/L, respectively). Allergic rhinitis (15.0%) and asthma (13.5%) were among the most frequent comorbid respiratory and allergy diseases. Conclusions: This is the first Spanish study reporting the overall diagnosed prevalence for a large-scale adolescent (12 to 17 years old) cohort from Catalonia. It provides new and robust evidence of AD’s prevalence and related characteristics in this region.