[en] Ni-doped KTaO₃ single crystals were grown by a self-flux method. Optical absorption and electron spin resonance (ESR) were measured at room temperature; four absorption bands and tetragonal ESR centers, caused by low-spin Ni³⁺ ions substituting for K⁺ ions, were observed. Real and imaginary parts of the dielectric constant, ε' and ε'', respectively, were measured in the temperature range of 10-470 K and in the frequency range of 30 Hz-10 MHz. Three dielectric relaxation modes exist at three different temperature regions: approximately 10-30, 170-270, and 350-450 K. The first relaxation at low temperatures is due to the ferroelectric relaxor, and the second and third ones are of the simple Debye type with activation energies of 0.38 and 0.51 eV, respectively. Some discussions are presented regarding the origin of these dielectric relaxation modes. (author)