Although it is well known that water is essential for human homeostasis and survival, only recently have we begin to understand its role in the maintenance of cognitive function. Objective: To investigate the relationship between hydration status and cognitive function among critically ill geriatric patients. Methodology: Thirty critically ill geriatric patients selected from the Intensive Care Units of the Alexandria Main University Hospital were included in the study. Three tools were used to collect the data; Assessment of hydration status of critically ill geriatric patients, The Mini- Mental State Examination, and Intensive Care Delirium Screening Checklist. Results: Of the total subjects, 70.0% had fluid volume deficit. Hemodynamic markers as temperature, heart rate, and central venous pressure were statistically significant different with hydration alteration. In addition to, physical markers as distention of neck vein, eye, lip & tongue conditions. 30.0% of the subjects had a moderate cognitive impairment and 50.0% had delirium with a statically significant difference between hydration alteration and cognitive impairment and delirium p= (.033, .047) respectively. Conclusion & recommendations: Dehydration or fluid volume deficit is prevalent in the critically ill geriatric patients. Additionally, cognitive impairment and delirium are more common in dehydrated patients than overhydrated patients. These findings suggest that critically ill geriatric patients should be continuously assessed for the presence of hydration status alterations (especially fluid volume deficit), cognitive impairment and delirium.