A bulk-acoustic wave resonator includes a substrate; a membrane layer forming a cavity with the substrate; a first electrode at least partially disposed on an upper portion of the cavity including an end portion that is thicker than other portions of the first electrode; an insertion layer including a first portion disposed adjacent to from the end portion of the first electrode and a second portion disposed on an upper portion of the first electrode; a piezoelectric layer disposed to cover the insertion layer; and a second electrode disposed on an upper portion of the piezoelectric layer.