This paper presents a new set of boundary conditions to be enforced on an elastodynamic wave interacting with a nonlinear interface. They are formulated within the framework of the quasi‐static approximation, and model the effect of defects on the interface properties by means of two distributions of nonlinear springs. The nonlinear terms are derived from the micromechanics of two rough surfaces in contact. Numerical results are presented both for normal and oblique incidence. © 2003 American Institute of Physics