An analytical closed-form solution has been developed for steady-state lateral response of concrete-face rockfill dams built in rectangular canyons. The canyon is assumed to be rigid, while the dam is idealized as a three-dimensional linearly-hysteretic elastic body deforming in shear and bending. Both free and base induced oscillations are studied for various canyon geometries. A parametric study of the effects of the canyon narrowness and the dam slope on the response is undertaken. Finally, a more rigorous numerical formulation is used for verification of the closed-form solution.