This adult cohort determined the incidence and patients' short-term outcomes of severe traumatic brain injury (sTBI) in Switzerland and age-related differences. A prospective cohort study with a follow-up at 14 days was performed. Patients ≥16 years of age sustaining sTBI and admitted to 1 of 11 trauma centers were included. sTBI was defined by an Abbreviated Injury Scale of the head (HAIS) score >3. The centers participated from 6 months to 3 years. The results are presented as percentages, medians, and interquartile ranges (IQRs). Subgroup analyses were performed for patients ≤65 years (younger) and >65 (elderly). sTBI was observed in 921 patients (median age, 55 years; IQR, 33-71); 683 (74.2%) were male. Females were older (median age, 67 years; IQR, 42-80) than males (52; IQR, 31-67; p<0.00001). The estimated incidence was 10.58 per 100,000 inhabitants per year. Blunt trauma was observed in 879 patients (95.4%) and multiple trauma in 283 (30.7%). Median Glasgow Coma Score (GCS) on the scene was 9 (IQR 4-14; 8 in younger, 12 in elderly) and in emergency departments 5 (IQR, 3-14; 3 in younger, 8 in elderly). Trauma mechanisms included the following: 484 patients with falls (52.6%; younger, 242 patients [50.0%]; elderly, 242 [50.0%]), 291 with road traffic accidents (31.6%; younger, 237 patients [81.4%]; elderly, 54 [18.6%]), and 146 with others (15.8%). Mortality was 30.2% (24.5% in younger, 40.9% in elderly). Median GCS at 14 days was 15 (IQR, 14-15) without differences among subgroups. Estimated incidence of sTBI in Switzerland was low, age was high, and mortality considerable. The elderly had higher initial GCS and a higher death rate, but high GCS at 14 days.