Potassium niobate (KNbO3) nanowires were synthesized by a hydrothermal method. X-ray diffraction and Raman spectroscopy indicated the nanowires are well-crystallized with orthorhombic structure. High resolution transmission electron microscopy showed the KNbO3 nanowires grew along the [110] direction. The photocatalytic water splitting results over KNbO3 nanowires showed that the rate for H2 evolution from aqueous CH3OH solutions was up to 5.17 mmol/h/gcat. To the best of our knowledge, the photocatalytic water splitting activity of the prepared nanowires is the highest in all KNbO3 materials.