The effect of lanthanum concentration on dielectric relaxation behavior in lead zirconate stannate titanate antiferroelectric ceramics was investigated by X-ray diffraction technique, dielectric spectroscopy, Sawyer–Tower polarization methods, and thermally stimulated depolarization current (TSDC) measurements. The tetragonal phase was found to be stabilized from the rhombohedral phase by lanthanum substitution. As the lanthanum content was increased, the dielectric constant decreased gradually over the measuring temperature range. As a result of polarization–electric field hysteresis loops, the maximum polarization decreased and the switching field increased with the increase of lanthanum content. Investigations of TSDC measurements revealed that the peak is due to space-charge polarization. It is suggested to be associated with the relaxation of oxygen vacancies.