The adsorption properties of water molecules on TiO₂ nanotubes (TiO₂NT) and the interaction mechanisms between water molecules are studied by first principles calculations. The adsorption preferences of water molecules in molecular or dissociated states on clean and H-terminated TiO₂NT are evaluated. Adsorption of OH clusters on (0, 6) and (9, 0) TiO₂ nanotubes are first studied. The smallest adsorption energies are -1.163 eV and -1.383 eV, respectively, by examining five different adsorption sites on each type of tube. Eight and six adsorption sites were considered for OH adsorbtion on the H terminated (0, 6) and (9, 0) nanotubes. Water molecules are reformed with the smallest adsorption energy of -4.796 eV on the former and of -5.013 eV on the latter nanotube, respectively. For the adsorption of a single water molecule on TiO₂NT, the molecular state shows the strongest adsorption preference with an adsorption energy of -0.660 eV. The adsorption of multiple (two and three) water molecules on TiO₂NT is also studied. The calculated results show that the interactions between water molecules greatly affect their adsorption properties. Competition occurs between the molecular and dissociated states. The electronic structures are calculated to clarify the interaction mechanisms between water molecules and TiO₂NT. The bonding interactions between H from water and oxygen from TiO₂NT may be the reason for the dissociation of water on TiO₂NT.