A bulk-acoustic wave resonator includes a substrate; a lower electrode formed on the substrate, and at least a portion of the lower electrode is formed on a cavity; a piezoelectric layer formed on the lower electrode; an upper electrode formed on the piezoelectric layer; a membrane layer formed below the lower electrode and forming the cavity together with the substrate; and a protruding portion formed on the membrane layer and further formed in the cavity in a direction that extends away from the membrane layer.