The prevalence of hypertonic dehydration (HD) among community-dwelling and hospitalized populations has been evaluated. However, to our knowledge, no study had previously focused on older patients admitted to the emergency department (ED). The present study aimed to evaluate the prevalence, risk factors and short-term outcomes of HD among Thai older patients admitted to the ED.Methods: This was a prospective cross-sectional study at one urban ED in Thailand. Patients aged ≥65 years who were admitted to the ED were enrolled into the study. Data including clinical hydration status, Charlson Comorbidity Index (CCI) score, activities of daily living score, current use of medications, laboratory examination results and serum osmolarity level were collected. HD was defined as a serum osmolarity level of >300 mOsm/kg. The short-term outcomes were a 30-day ED revisit, hospital readmission and mortality rates.Results: In total, 80 (21.6%) of 370 patients presented with HD. A CCI score of ≥5 was found associated with HD among older patients (adjusted odds ratio: 1.82; 95% confidence interval: 1.03-3.21). The ED revisit rates were 18.1% in the dehydrated group and 10.9% in the non-dehydrated group. The hospital readmission rates were 8.3% in the dehydrated group and 10.6% in the non-dehydrated group. Furthermore, the 30-day mortality rates were 6.9% and 5.3% in the dehydrated and non-dehydrated groups, respectively.Conclusions: One-fifth of older patients admitted to the ED presented with HD. A CCI score of ≥5 was considered a risk factor of HD. Moreover, further studies should focus on the long-term outcomes of HD and risk reduction.