In recent decades, expanding research on topology optimization (TO) across engineering fields has enabled a wide range of innovative, high-performance designs while minimizing material consumption. This demonstrates that TO is a powerful tool for promoting sustainable engineering practices through reduced material and energy usage. Despite these advantages, the application of TO to geotechnical engineering remains limited. The inherent complexities of soil–structure interaction and nonlinear material behavior pose significant challenges to the application of TO, highlighting the need for further research. In this context, the present work investigates the application of topology optimization to geotechnical design using the Floating Projection Topology Optimization (FPTO) method. The FPTO method is a continuous approach that employs a numerical projection scheme to the design variables to impose 0/1 constraints, thereby avoiding the need for any material penalization schemes. The semi-infinite soil domain is modeled through a coupled finite element–boundary element formulation, allowing an efficient and consistent representation of soil–structure interaction effects. The structures are optimized using a multi-objective formulation that maximizes stiffness while minimizing the maximum stress in the coupled soil–structure system.Representative geotechnical cases, including raft foundations, bridge foundations, and tunnel systems, are analyzed to evaluate the performance and robustness of the proposed framework. The results demonstrate the potential of FPTO to generate efficient geotechnical structural layouts contributing to the advancement of topology optimization in underground and foundation engineering applications.