Background: Dementia constitutes a public health hazard in developing countries. The aim of this study was to evaluate the prevalence of dementia and its associated factors in older hospitalized patients. Methods: The participants of this cross-sectional study consisted of older patients admitted to medical wards in Rasoul-e Akram hospital in Tehran, Iran. Mini-Mental State Examination, Mini-Cog test, Geriatric Depression Scale, Activities of Daily Living-Instrumental Activities of Daily Living (ADL-IADL) scale, and socioeconomic questionnaires were used. Results: A total of 205 elderly inpatients were included. The mean age was 71.33 ± 7.35 years; 63.4% of the participants had normal cognitive function, while 36.6% had some degree of cognitive impairment. There was a statistically significant relationship between gender, age, number of children, and occupation and the prevalence of dementia. Conclusion: Appropriate cognitive screening of older patients upon admission to hospitals could help identify potential adverse events and enhance the quality of care for patients with comorbid dementia.