The temperature dependence of the off-centre Li position and surrounding lattice polarization in Li:KTaO₃ is calculated within the quasiharmonic approximation using a nonlinear polarizability model. The Li displacement is found to be almost independent of temperature. Both the total dipole moment of the region where the Li dipole moment is reinforced by the lattice polarization and the size of this region become smaller with decreasing temperature. The Li displacement as well as the total dipole moment are described quantitatively by this model, provided that the Li concentration is below approximately 1%.