In this paper, we applied an incompressible Smoothed Particle Hydrodynamics (SPH) method to investigate the impact of solitary waves on seawalls, especially movable seawalls. The SPH method is a mesh-free numerical approach particularly suitable for dealing with large free surface deformations and complex fluid-structure interactions. The incompressible SPH (ISPH) method solves the pressure field using the pressure Poisson equation (PPE), rather than relying on the equation of state. It has the advantage of producing more stable and accurate pressure fields and impact forces on structures. We first applied the model to simulate the solitary wave propagation and runup against a fixed vertical wall. The computations compared well with previous experimental and numerical results. Then, the solitary wave impact on a movable structure was investigated by replacing the fixed wall with a spring-controlled seawall subject to different spring stiffness and mass settings. Particular attention was paid to the prediction of the seawall movement, wave runup height and hydrodynamic loading. The incident wave height was found to be the dominant factor for the movable seawall movement during and immediately after the wave crest arrival at the seawall. Other factors, such as the seawall mass and spring stiffness, become important to the seawall's responses only after the maximum impact.