Water adsorbed on carbon nanotube (CNT) surfaces is known to have unique properties, however, the structure and dynamics of adsorbed water on CNT have been unclarified. We investigated the temperature dependence of the structure and rotational dynamics of adsorbed water on carbon nanotube surfaces using a classical molecular dynamics simulation. At a specific adsorption amount and temperature, a rhombic ice structure and a polygon structure that includes pentamers and hexamers coexist in the adsorbed water. Rotational dynamics analysis indicates that the rhombic ice exhibits solid-like behavior and that the polygon structure exhibits liquid-like behavior. Their coexistence is regarded as a solid–liquid crossover.