Current prosthetic feet are primarily optimized for sagittal-plane stiffness and energy return, while their rotational compliance in the transverse plane (inversion–eversion) remains limited. This restriction can reduce adaptability to uneven terrain and compromise user comfort. This work presents a first-step multidisciplinary topology optimization framework for designing flexible modular metamaterials that enhance transverse rotational compliance while maintaining lightweight and load-bearing performance. The proposed approach focuses on the topology optimization of metamaterial modules with anisotropic stiffness characteristics, enabling controlled rotational flexibility about the transverse axis. As an initial investigation, the framework is formulated under linear elastic assumptions with prescribed vertical loading and transverse rotational moments, allowing systematic evaluation of stiffness–compliance trade-offs without the added complexity of full contact or gait dynamics. At the module level, material distributions are optimized to balance weight reduction, vertical stiffness, and rotational compliance, subject to volume and manufacturability constraints relevant to additive manufacturing. At the assembly level, optimized modules are arranged within a simplified prosthetic foot geometry to achieve region-specific mechanical responses, including compliant rotation in the hindfoot and stable support in the midfoot and forefoot. Numerical results demonstrate that the optimized modular metamaterial designs exhibit significantly reduced transverse rotational stiffness compared to conventional homogeneous structures, while preserving structural integrity and achieving notable mass reduction. The study shows that modular topology-optimized architectures can effectively decouple vertical stiffness and transverse rotational compliance. Significance: This work establishes a rational and computationally efficient foundation for adaptive prosthetic foot design and provides a scalable pathway toward more realistic nonlinear and biomechanical analyses in future studies.