A piezoelectric substrate includes: a substrate; a first electrode formed on the substrate; and a piezoelectric layer formed on the first electrode and containing potassium, sodium, and niobium. A full width at half maximum of an X-ray intensity peak on a plane (100) of the piezoelectric layer in a Psi axis-direction scan result of an X-ray diffraction measurement in which a surface of the piezoelectric layer is irradiated with X-rays at an angle of 54.74° from a direction perpendicular to the surface is more than 0° and 1.2° or less.