Twenty-six groundwater samples were collected From Tanegashima, Yakushima, and Nakanoshima islands between July 27 and August 2, 2003. The interpretation of chemical analyses suggested that the groundwater on the islands have been affected by sea salt (ss). Chlorine (Cl^-) concentrations on Yakushima, which were inversely related to elevation, were considerably lower than those on Nakanoshima and Tanegashima, reflecting higher annual rainfall on Yakushima. However, groundwater samples from Yakushima had a higher ratio of ss components—more than 50% (except one sample). Non-sea salt (nss) components of the groundwater on Tanegashima, Yakushima, and Nakanoshima were found to be affected mostly by the chemical weathering of plagioclase feldspars. The groundwater on Tanegashima was also thought to be affected by ion exchange between clay minerals and Ca²⁺. The effects of sulfuric acid and carbonic acid on chemical weathering were nearly equivalent on Nakanoshima. Therefore, the average concentrations of nssSO₄^2- and dissolved SiO₂ were remarkably higher than those for Yakushima and Tanegashima. The arithmetic mean concentration of nssK⁺ on Yakushima was high even though the total concentration of all nss components was low. The nssK⁺ probably originates from chemical weathering of K-feldspar in granite. The total concentration of nss components on Yakushima was remarkably lower than that on Tanegashima or Nakanoshima. Low NO₃^--N concentrations in these mountainous and forested islands reflect a low load from anthropogenic activities.