A diffusion theory method is developed for synthetic acceleration of nodal Sn calculations in multidimensional Cartesian geometries. The diffusion model is derived from the spatially continuous diffusion equation by applying spatial approximations that are P1 expansions of the corresponding approximations made in solving the transport equation. The equations of the diffusion model are formulated in a way that permits application of existing and highly efficient nodal diffusion theory techniques to their numerical solution. Test calculations for several benchmark problems in X-Y geometry are presented to illustrate the efficiency and stability of the acceleration method when applied to a “constant-linear” nodal transport approximation. The method is shown to yield point-wise flux convergence of 10-4 in fewer than ten synthetic iterations for all problems considered and to require substantially less computational effort than unaccelerated solutions.