Aim Delirium, a common disorder in hospitalized older patients, frequently results in unfavorable consequences. Previous studies in different settings have provided conflicting results regarding clinical outcomes and mortality. We aimed to study three clinical outcomes – length of stay (LOS), in‐hospital mortality and 3‐month mortality – among delirious Thai older patients. Methods A prospective observational study was carried out in a university hospital in Thailand. All patients aged older than 70 years admitted to general medical wards were included. Delirium assessments were undertaken within the first 24 h of admission and serially until discharge. Subsequent assessments were carried out at 3 months after discharge. Delirium was diagnosed based on the Diagnostic and Statistical Manual of Mental Disorders fourth edition criteria. Factors associated with mortality were determined by using logistic regression models. Results LOS was significantly longer in the delirium group (10 and 8 days, P = 0.001). Furthermore, the delirium group had higher in‐hospital and 3‐month mortality (P < 0.001). Factors significantly associated with in‐hospital mortality in multivariate analysis were age more than 80 years (AOR 2.74, 95% CI 1.05–7.15), malignancy (AOR 3.11, 95% CI 1.16–8.33), severe illness (AOR 3.75, 95% CI 1.38–10.20) and delirium (AOR 7.34, 95% CI 1.51–35.69). Delirium remained a strong predictor for 3‐month mortality in multivariate analysis with AOR of 3.33 (95% CI 1.45–7.62) Conclusions Delirium was associated with prolonged hospital‐stay and was the strongest predictor for mortality among older hospitalized patients. It requires serious attention from physicians, healthcare administrators and policy makers to implement an appropriate management plan for this high‐burden issue. Geriatr Gerontol Int 2013; ●●: ●●–●●.