This work focuses on the development of a new semi- implicit SPH scheme for the shallow water equations, 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. This paper proposes, a new semi-implicit SPH scheme, which leads to an unconditionally stable method. To this end, 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 easily be solved by a matrix- free conjugate gradient method. Once the new free surface location is known, the velocity at the new time level can directly be computed and the particle positions can subsequently be updated. A simple and yet non-trivial 1D test problem for the 1D shallow water equation is presented.