This paper presents the nonlinear response of an earthfill dam subjected to spatially varying ground motion, which includes the wave-passage, incoherence and site response effects. An equivalent linear method is considered in the nonlinear response formulation. The importance of the site response effect, which arises from the difference in the local soil conditions at different support points of the structure, is investigated particularly. The mean of maximum and variance response values obtained from a spatially varying ground motion case are compared with those of specialized ground motion models. It is observed that the variation in local soil conditions has important effects on the nonlinear response of earthfill dams.