Rare earth element (Sm)-doped potassium sodium niobate (KNN)-based ceramics are fabricated using spark plasma sintering method and their properties are investigated. The results show that all the samples crystallize in a typical perovskite structure with a single orthorhombic phase. With increasing the Sm doping, the ceramics gradually shift toward the relaxor ferroelectric state and the value of dielectric loss angle tangent (tanδ) is smaller than 0.05 for x ≤ 0.003 ceramic samples. Meanwhile, the optimized piezoelectric charge coefficient d₃₃ = 128 pC/N, and piezoelectric voltage coefficient g₃₃ = 18.9 × 10⁻³ Vm/N are obtained when x = 0.001. Compared with the undoped sample, the d₃₃ of x = 0.001 ceramics has been significantly enhanced by 28%. The positron annihilation lifetime results indicate that the main defect types in the ceramics are the A-site vacancies and defect dipoles. Based on the aforementioned results, the optimized piezoelectric performance and the lowest defect dipoles concentration in x = 0.001, may be attributed to the low internal oxygen vacancy concentration in it. This work may provide insights for the further study of KNN-based piezoelectric ceramics.