An acoustic wave device includes a silicon substrate, a piezoelectric layer, and an IDT electrode. Each of the silicon substrate and the piezoelectric layer includes first and second opposed main surfaces. The IDT electrode is on the first main surface of the piezoelectric layer, and includes first and second electrode fingers. When a wavelength of an acoustic wave determined by an electrode finger pitch of the IDT electrode is denoted as λ, a distance between the first main surface of the silicon substrate and the second main surface of the piezoelectric layer in a thickness direction of the silicon substrate is less than about 0.84λ. The first main surface of the silicon substrate is rougher than the first main surface of the piezoelectric layer.