This work focuses on the extension of the semi- implicit SPH scheme to two dimensions for the shallow water equations. The scheme in was first presented at last year’s SPHERIC workshop, following the semi-implicit finite volume and finite difference approach of Casulli. In standard explicit numerical methods, there is often a severe limitation on the time step due to the stability restriction imposed by the CFL condition. To this end, a new semi-implicit SPH scheme is derived, which leads to an unconditionally stable method. The discrete momentum equation is substituted into the discrete continuity equation to obtain a symmetric positive definite linear system for the free surface elevation. The resulting system can be easily solved by a matrix-free conjugate gradient method. Once the new free surface location is known, the velocity at the new time level can be directly computed and the particle positions can subsequently be updated. The method is validated on a smooth inviscid hydrostatic free surface flow for the two dimensional shallow water equations.