A bulk-acoustic resonator includes: a substrate; a first electrode disposed on the substrate; a piezoelectric layer at least partially covering the first electrode, and including a flat portion disposed in a central region, and an extension portion disposed outside the flat portion and having at least one step portion; an insertion layer disposed on the extension portion; and a second electrode disposed on upper portions of the insertion layer and the piezoelectric layer. The extension portion includes at least one first surface and at least one second surface disposed below an upper surface of the flat portion, and a connection surface connecting an upper surface of the flat portion to the at least one first surface or the at least one second surface, or connecting first surfaces among the at least one first surface to each other or second surfaces among the at least one second surface to each other.