Weight reduction is a major issue in designing high-speed train components to reduce train power consumption. In this study, an automatic coupler that is being developed for the high-speed train in Indonesia is selected for weight reduction by using topology optimization. The simulation model is referring to an automatic coupler model for light trains that had been developed previously. The main component of the automatic coupler selected for optimization is the hooked plate. Longitudinal train force analysis was carried out to obtain tensile and compressive forces on the coupler. Iteration of retaining every 10% is performed on topology optimization simulation. The new design is made with an over-designed form by giving excess material to the geometry. This study indicates that the mass reduction on the hooked plate is 24% form based on the von Mises stress distribution under high-speed train loading. In total, the new design contributes to reducing the mass of the automatic coupler by 4%.