The effects of doping niobium and scandium in Pb[(Sc1/2Nb1/2)0.575Ti0.425]O3 piezoelectric polycrystalline ceramics were investigated. Complete single-phase perovskite with high density was obtained with all samples fired at 1275?C. Niobium doping decreased the grain size, enhanced the maximum dielectric constant value and increased the electrical resistivity by two orders of magnitude compared with nondoped samples. Large electromechanical coupling factors of kp=69%, kt=52% and k33=76%, which are almost equivalent to value for the best PZT-based piezoelectric ceramics, were obtained in niobium-doped specimens. The material has the largest electromechanical coupling factor of all relaxor-lead titanate binary ceramics systems so far reported.