An acoustic resonator includes a substrate and a resonant portion. The resonant portion has a central portion in which a first electrode, a first piezoelectric layer, a second piezoelectric layer, and a second electrode are stacked in order on the substrate, and an extension portion extending outwardly from the central portion and including an insertion layer. A ratio of an average thickness of the first piezoelectric layer to an average thickness of the second piezoelectric layer is 18.4% to 40%.