An acoustic wave device includes a high-acoustic-velocity support substrate, a low-acoustic-velocity film provided on the high-acoustic-velocity support substrate, a piezoelectric layer provided on the low-acoustic-velocity film, and an IDT electrode provided on the piezoelectric layer. An acoustic velocity of a bulk wave propagating through the high-acoustic-velocity support substrate is higher than an acoustic velocity of an acoustic wave propagating through the piezoelectric layer. An acoustic velocity of a bulk wave propagating through the low-acoustic-velocity film is lower than an acoustic velocity of a bulk wave propagating through the piezoelectric layer. The low-acoustic-velocity film has a first portion and a second portion that is located closer to the high-acoustic-velocity support substrate than the first portion. The first and second portions include the same or similar materials. A density in the first portion of the low-acoustic-velocity film and a density in the second portion of that are different.