Studies of nonlinear acoustic interactions in superfluid helium at temperatures below 0.2 K have culminated in the construction of an all‐acoustic parametric amplifier at gigahertz frequencies. This device represents the shortest wavelength parametric amplifier ever made, with signal wavelengths shorter than 1000 A˚ and pump wavelengths shorter than 600 A˚. In the experiment, plane waves are mixed at a predefined angle in the superfluid helium. Two gain regimes are observed. The first regime is a noncollinear phase‐matched process, in which the signal phonon stimulates decay of the pump phonon to create gain at the signal frequency. The second regime is a four‐phonon collinear process, in which gain on the signal is created by the second harmonic of the pump. This four‐phonon process is unusual and is shown to be a combination of three‐phonon processes wherein the lack of conservation of energy and momentum in one process is mostly compensated by the other.