Submarine or/and subaerial landslides sometimes cause a series of hazardous events, such as severing of submarine cables and pipelines, obstructing the operation of resource development facilities, and large-scale tsunamis. The damage caused by such a landslide-induced tsunami has been considered globally, and the resultant casualties has also been significant. However, the process of such an event is not well understood due to multiple physical phenomena simultaneously interacting (the collapse of the seabed, the interaction between soil and water). Conventional approaches lack the ability of predicting the coupling behavior between soil and seawater with high accuracy. In order to evaluate such complex interaction, we have proposed a novel numerical method, called MPM-FEM hybrid method, in which the material point method (MPM) is applied to the governing equation of the solid phase by the Lagrangian description, whereas the stabilized finite element method (FEM) is applied to that of the liquid phase in the Eulerian frame. Nevertheless, fully 3D computations to simulate the propagation of offshore waves require significantly high computational costs and are not suitable for large-scale and long-duration simulations. In this context, the 2D shallow-water equation has been widely used for simulating large-scale offshore wave propagation because of its relatively lower computational costs. Against this background, in this study, we devise a strategy to realize the coupling between 3D and 2D domains with a view to simulating the entire process of a tsunami induced by submarine landslides in an offshore region. Several numerical examples are presented to demonstrate the performance of the proposed method.