A series of yttria-stabilized zirconia single-crystals doped with 0.000–0.250 mol% Tb₄O₇ was prepared by the optical floating-zone method. As shown by XRD and Raman spectroscopy, all of the crystals had a cubic-phase structure. These were initially orange–yellow in color, which is indicative of the presence of Tb⁴⁺ ions, but they then became colorless after being annealed in a H₂/Ar atmosphere as a result of the reduction of Tb⁴⁺ to Tb³⁺. The absorption spectra of the unannealed samples show both the 4f ⁸→4f ⁷5d¹ transition of Tb³⁺ ions and the Tb⁴⁺ charge-transfer band. In addition, the transmittance of the crystals was increased by annealing. Under irradiation with 300 nm of light, all of the single-crystal samples showed seven emission peaks in the visible region, corresponding to the decay from the ⁵D_3,4 excited state of Tb³⁺ to the ⁷F_J (J = 6–0) states. The most intense emission was at 544 nm, which corresponds to the typical strong green emission from the ⁵D₄→⁷F₅ transition in Tb³⁺ ions.