An acoustic wave device includes a mounting substrate; an acoustic wave element on one major surface of the mounting substrate in its thickness direction, and a bump between the acoustic wave element and the mounting substrate. The acoustic wave element includes a support substrate including an air gap, a piezoelectric layer stacked on the support substrate and including an overlap region at least partially overlapping the air gap as viewed in the stacking direction, and a functional electrode located in the overlap region of the piezoelectric layer. The mounting substrate includes a metal portion. A fixed capacitance generated between the acoustic wave element and the mounting substrate is not less than a variable capacitance generated between the acoustic wave element and the mounting substrate.