Hot spring waters in Shimogamo, Izu Peninsula, Japan, were analyzed for the stable isotopes of boron as well as for chemical compositions. The boron isotopic ratios, ¹¹B/¹⁰B, of the waters range from 4.190 to 4.216, corresponding to the variation in δ¹¹B from +36.2 to +42.6 permil. These values are very high compared to those of hot spring waters in other areas on the Japanese islands and close to that of sea water. The results of the isotopic and chemical analyses indicate that the Shimogamo hot spring waters are mixtures of surface water and a deep geothermal brine directly derived from sea water through the interaction with heated silicate rocks.