A method is needed for measuring the water content in the stratum corneum (SC), the outermost layer of human skin, in a way that is less affected by external formulations on the skin, such as cosmetics. Here, we evaluated continuous-wave terahertz attenuated total reflection (THz-ATR) spectroscopy for measuring the water content in the SC with external formulations applied. THz waves were selected because they are strongly absorbed by water but are transmitted through various materials. We found that the water content in the SC sample was strongly correlated with the ATR signal, which represents THz wave absorption by the sample, and there was little THz wave absorption by ingredients in the external formulations. Moreover, THz-ATR spectroscopy of moisturized skin provided results consistent with the conventional method. Taken together, THz-ATR spectroscopy is expected to enable measurement of the water content in the SC with less influence from materials on the skin.