Objective Stratum corneum hydration is an important indicator of skin conditions such as xerosis and maceration. This quasi-experimental study assessed the reliability and validity of a portable device for stratum corneum hydration measurement designed for easy clinical use. Method The participants were university students（n=22, 12 male and 10 female）. Hydration of the flexor surface of the forearm and the ventral surface of the heel were measured in triplicate by four raters. Hydration of the forearm was measured again using a portable device（Mobile Moisture HP-10N™）and a standard device（Corneometer CM825®）10 minutes after application of a moisturizer. Repeatability, concurrent validity between the devices, known-groups validity between the sites, and responsiveness to the moisturizer were then evaluated. Results The intraclass correlation coefficients of the triplicate measurements ranged from 0.88-0.97, and the intra-rater reliability was almost perfect. The rank correlation coefficients between two devices were 0.87 for the arm and 0.93 for the heel（both p<0.001）but the portable device showed significantly higher values than the standard device for both sites（p<0.001）. The portable device also yielded a higher hydration value for the forearm than for the heel（47.6［SD7.3］vs 33.6［13.7］, p=0.045）, and the difference between these two values was equivalent in degree to that between the corresponding values obtained with the standard device. The portable device was able to detect an increase in hydration due to the application of the moisturizer, and showed a positive correlation with the results obtained from the standard device in terms of the ifference of hydration, its ratio, and the area under the curve. This study demonstrated that the portable device produced an adequate level of repeatability, concurrent validity for relative values, known-groups validity, and responsiveness.