We report the synthesis of KNbO3 nanowires (NWs) with a monoclinic phase, a phase not observed in bulk KNbO3 materials. The monoclinic NWs can be synthesized via a hydrothermal method using metallic Nb as a precursor. The NWs are metastable, and thermal treatment at similar to 450 degrees C changed the monoclinic phase into the orthorhombic phase, which is the most stable phase of KNbO3 at room temperature. Furthermore, we fabricated energy-harvesting nanogenerators by vertically aligning the NWs on SrTiO3 substrates. The monoclinic NWs showed significantly better energy conversion characteristics than orthorhombic NWs. Moreover, the frequency-doubling efficiency of the monoclinic NWs was similar to 3 times higher than that of orthorhombic NWs. This work may contribute to the synthesis of materials with new crystalline structures and hence improve the properties of the materials for various applications.