A theory is developed for acoustic radiation pressure exerted by a plane wave on a perfect absorber, using Euler's momentum theorem. For the case of an open vessel, the radiation pressure P equals the mean energy density E even when nonlinearities of the medium and distortion of wave form are taken into account. For a closed vessel P equals [1 + 12(B/A)]E. The term B/A describes the non-linearity in the pressure-density relation for the medium and equals γ−1 for the case of an ideal gas under adiabatic conditions.