BACKGROUND: Spain has been one of the countries most affected by the COVID-19 pandemic. OBJECTIVE: To create a registry of patients with COVID-19 hospitalized in Spain, in order to improve our knowledge of the clinical, diagnostic, therapeutic, and prognostic aspects of this disease. METHODS: A multicentre retrospective cohort study, including consecutive patients hospitalized with confirmed COVID-19 throughout Spain. Epidemiological and clinical data, additional tests at admission and at seven days, treatments administered, and progress at 30 days of hospitalization were collected from electronic medical records. RESULTS: Up to June 30th 2020, 15,111 patients from 150 hospitals were included. Their median age was 69.4 years (range: 18-102 years) and 57.2% were male. Prevalences of hypertension, dyslipidemia, and diabetes mellitus were 50.9%, 39.7%, and 19.4%, respectively. The most frequent symptoms were fever (84.2%) and cough (73.5%). High values of ferritin (73.5%), lactate dehydrogenase (73.9%), and D-dimer (63.8%), as well as lymphopenia (52.8%), were frequent. The most used antiviral drugs were hydroxychloroquine (85.6%) and lopinavir/ritonavir (61.4%); 33.1% developed respiratory distress. Overall mortality rate was 21.0%, with a marked increase with age (50-59 years: 4.7%, 60-69 years: 10.5%, 70-79 years: 26.9%, >/=80 years: 46.0%). CONCLUSIONS: The SEMI-COVID-19 Network provides data on the clinical characteristics of patients with COVID-19 hospitalized in Spain. Patients with COVID-19 hospitalized in Spain are mostly severe cases, as one in three patients developed respiratory distress and one in five patients died. These findings confirm a close relationship between advanced age and mortality.