Because of growing environmental concerns, the development of lead-free piezoelectric materials with enhanced properties has become of great interest. Here, we report a giant piezoelectric coefficient (d(33)) of 550 pC/N and a high Curie temperature (T-C) of 237 degrees C in (1-x-y)K(1-w)Na(w)Nb(1-z)Sb(z)O(3-)xBiFeO(3-)yBi(0.5)Na(0.5)ZrO(3) (KNwNSz-xBF-yBNZ) ceramics by optimizing x, y, z, and w. Atomic-resolution polarization mapping by Z-contrast imaging reveals the intimate coexistence of rhombohedral (R) and tetragonal (T) phases inside nanodomains, that is, a structural origin for the R T phase boundary in the present KNN system. Hence, the physical origin of high piezoelectric performance can be attributed to a nearly vanishing polarization anisotropy and thus low domain wall energy, facilitating easy polarization rotation between different states under an external field.