Ferroelectric lead zirconate titanate (PZT, PbZr_1−xTi_xO₃) dominates the piezoelectric ceramic market because of its high piezoelectric coefficient, high thermal stability and low processing cost. Doping of PZT with lanthanum has been widely used to develop advanced piezoelectric ceramics with improved piezoelectric properties. However, the effects of La doping are ambiguous in the literature, and associated mechanisms are unclear. Here, we prepared La-doped PZT (PLZT) ceramics with different concentrations of La by the conventional solid-state reaction method. The PLZT ceramics have a perovskite structure, where tetragonal (P4mm) and monoclinic (C1m1) phases coexist. Upon La doping, the Curie point (T_c), ferroelectric lattice distortion, and concentration of oxygen vacancies decrease. Under an applied electric field, both domain switching and field-induced transition are observed. The remnant polarization (P_r) of the doped PLZT ceramics is greater than that of the starting PZT ceramics, and the increased P_r is attributed to the substitution of reduced oxygen vacancies. With increasing temperature, the piezoelectric coefficient (d₃₃) of the La-doped ceramics increases due to the increase in permittivity. The prevailing effect of high permittivity is related to the high d₃₃ value (~375 pC/N) of the x = 0.5 ceramics. These findings reveal the mechanisms associated with La doping in PZT ceramics, thereby providing a viable approach for developing advanced piezoelectric ceramics with high piezoelectric performance.