The ionic conductivity along the principal axes a, b, and c of the unit cell of the nonlinear-optical high-resistance KTiOPO4 single crystals (rhombic syngony, space group Pna21), which are as-grown and after thermal annealing in vacuum, has been investigated by the method of impedance spectroscopy. The crystals were grown from a solution-melt by the Czochralski method. The as-grown KTiOPO4 crystals possess a quasi-one-dimensional conductivity along the crystallographic c axis, which is caused by the migration of K+ cations: σ║c = 1.0 × 10–5 S/cm at 573 K. Wherein the characteristics of the anisotropy of ionic conductivity of the crystals is equal to σ║c/σ║a= 3 and σ║c/σ║b= 24. The thermal annealing at 1000 K for 10 h in vacuum increases the magnitude of σ║c of KTiOPO4 by a factor of 28 and leads to an increase in the ratio σ║c/σ║b= 2.1 × 103 at 573 K. A crystal-physical model of ionic transport in KTiOPO4 crystals has been proposed.