Axillary skin dryness is a typical physical symptom of dehydration. This study was conducted to examine whether measuring axillary moisture with a simple skin hydrometer could be used as an objective indicator for dehydration assessment by nurses. The subjects were 86 patients aged 65 years or older admitted urgently to our hospital. Physical findings included observation of oral dryness, skin turgor, and sunken eyes and measurement of capillary refill time and axillary moisture. A simple commercially available skin hydrometer was used to measure axillary moisture. Blood tests were conducted to measure serum sodium levels, blood glucose, blood urea nitrogen, creatinine, and estimated glomerular filtration rate. Plasma osmolality was then calculated. In the dehydrated group, axillary moisture was lower, capillary refill time was delayed, and blood test results showed elevated sodium, blood urea nitrogen, and creatinine levels. There was consistency between plasma osmolality, axillary moisture and blood test values. Although the measurement accuracy of the simple hydrometer has not been validated, the quantification of axillary moisture using this instrument could be a simple diagnostic tool enabling objective nursing assessment of dehydration.