Provided is a variable capacitance element that includes a dielectric layer whose relative dielectric constant varies in accordance with an applied voltage and that can achieve reduction in size of a piezoelectric substrate such as a piezoelectric resonant component and a tunable filter. A variable capacitance element (1) includes a piezoelectric substrate (2), a buffer layer (3) that is formed on the piezoelectric substrate (2) and has an orientation, a dielectric layer (4) that is formed on the buffer layer (3) and whose relative dielectric constant varies in accordance with an applied voltage, and a first electrode and a second electrode that are provided so as to be capable of applying an electric field to the dielectric layer (4).