Background Severe traumatic brain injury (STBI) is one of the major causes of death and disability worldwide. The incidence and risk factors of enteral nutrition (EN)-associated diarrhea in older patients with STBI remain unclear. Methods A cohort of adult STBI patients were retrospectively studied. The patients were stratified into an older group (>= 65years) and a young group (< 65 years). All patients received EN for at least 48 h. Demographic, clinical and nutritional data were collected for analysis. We utilize multiple logistic regression models to evaluate predictors of diarrhea. Results Among 292 patients with STBI aged 60.38 +/- 14.89 years (mean +/- standard deviation), 114 cases developed diarrhea, with an incidence of 39.04%. Older patients had a higher incidence of diarrhea than young patients (46.77% vs 33.33%, p = 0.020). Three variables were found to be significantly associated with diarrhea in young STBI patients. In contrast, five variables were significantly associated with this complication in older STBI patients, including acute physiology and chronic health evaluation II score (adjusted OR 1.134, 95% CI 1.019-1.272, p = 0.025), high-fat energy (adjusted OR 1.221, 95% CI 1.055-1.789, p = 0.025), EN duration (adjusted OR 1.105, 95% CI 1.005-1.223, p = 0.044), antibiotics total defined daily dose (DDDs) (adjusted OR 1.076, 95% CI 1.029-1.211, p = 0.039) and tube feeding of potassium (adjusted OR 2.525, 95% CI 1.031-6.450, p = 0.046). Conclusions Enteral nutrition-associated diarrhea was prevalent among STBI patients. Older STBI patients had a higher incidence of diarrhea and more risk factors than young patients. Early management of modifiable risk factors may help reduce the incidence of diarrhea.