A bulk-acoustic wave resonator includes: a substrate; a first electrode disposed on the substrate; a cavity disposed between the substrate and the first electrode; a piezoelectric layer covering at least a portion of the first electrode; a second electrode covering at least a portion of the piezoelectric layer; an insertion layer disposed between the first electrode and the piezoelectric layer; and a lower frame disposed in the cavity. At least a portion of the lower frame overlaps the insertion layer.