The influence of tetragonality, which is defined as the lattice parameter ratio c/a, on the tetragonal-to-monoclinic phase transformation during hydrothermal aging was investigated in yttria-stabilized zirconia coatings. The yttria content was adjusted in the range of 4–8 mass% (denoted as xYZ, where x= 4–8 and YZ represents the yttria-stabilized zirconia). The tetragonality of the zirconia in the as-sprayed coatings was less than that in the powders. To change the tetragonality for each yttria content, the coatings were annealed at 1273 K before aging. Without annealing, the phase transformation was prevented only in 8YZ. When annealing was applied, an increase of the tetragonality (i.e., recovery of the tetragonality) was observed, and transformation during hydrothermal aging was also suppressed in 6YZ. It was concluded that the increase in tetragonality that occurred without a change in the yttria content was suggested to be caused by the lattice relaxation of the tetragonal phase, and this relaxation is believed to cause a reduction of the critical yttria concentration, thus preventing the phase transformation.