The properties of nanoconfined and interfacial water in the proximity of hydrophobic surfaces play a pivotal role in a variety of important phenomena such as protein folding. Water inside single-walled carbon nanotubes (SWNTs) can provide an ideal system for investigating such nanoconfined interfacial water on hydrophobic surfaces, provided that the nanotubes can be opened without introducing excess defects. Here, we report a hydrophobic-hydrophilic transition upon cooling from 22 degrees C to 8 degrees C via the observation of water adsorption isotherms in SWNTs measured by nuclear magnetic resonance. A considerable slowdown in molecular reorientation of such adsorbed water was also detected. The observed transition demonstrates that the structure of interfacial water could depend sensitively on temperature, which could lead to intriguing temperature dependences involving interfacial water on hydrophobic surfaces.