Lead-free piezoelectric ceramics (K_0.474Na_0.474Li_0.052)(Nb_0.948Sb_0.052)O₃–x%Co₂O₃ [KNLNS–x%Co₂O₃] were prepared by the conventional solid-state sintering method. The effects of the Co₂O₃ addition (0.03–0.40 mol %) on the phase structure and piezoelectric properties of KNLNS ceramics were systematically studied. The phase structure of KNLNS–x%Co₂O₃ ceramics at room temperature is the coexistence of orthorhombic–tetragonal phases, and only a small amount of Co₂O₃ additives can diffuse into KNLNS lattices to form a stable solid solution. The density of ceramics increases gradually with the introduction of Co₂O₃. KNLNS–0.1 mol %Co₂O₃ lead-free ceramics exhibit optimum properties (d₃₃ = 262 pC/N, k_p = 0.40, ε_r = 1025, tan δ = 0.03 and T_c = 352°C), owing to the T_O–T near room temperature and the dense microstructure. The underlying physical mechanisms were clearly addressed. These results indicate that the KNLNS–x%Co₂O₃ ceramics are a promising candidate material for lead-free piezoelectric ceramics.