Using first-principles calculations, we show that substitution of sulfur (anion) in the oxygen site of tetragonal potassium niobate perovskite (KNbO3) leads to a highly polar and very stable ferroelectric (FE) state. It was found that KNbO2S possesses a spontaneous polarization of 0.80 C/m2, twice that of the mother FE KNbO3. This variation is due to the large out-of-plane displacements of the substituted anion with respect to the cations. The stability of the FE state is also maintained upon large in-plane tensile strain, and the fast decay rate of FE stability gives rise to an exponential increase in piezoelectric constant (d33).