Background: Customized temporomandibular joint (TMJ) prostheses enabled by CAD/CAM and additive manufacturing allow design flexibility in the sagittal curvature of the glenoid fossa. However, unlike standardized commercial systems, these patient-specific designs lack biomechanical guidelines, and the effect of sagittal curvature geometry on stress distribution and prosthetic kinematics remains unclear. Objective: To evaluate the biomechanical and kinematic influence of sagittal curvature radius in customized TMJ prostheses and identify geometries that reduce contact stresses while improving condylar translation. Methods: Three-dimensional finite element models were developed from computed tomography data of two female patients with distinct occlusal plane characteristics. Five sagittal curvature radii, from spherical to progressively elliptical profiles, were designed for the glenoid fossa. Patient-specific occlusal muscle vectors, prosthetic kinematic center, occlusal plane orientation, and unilateral molar loading of 250 N were incorporated. Linear elastic material models were applied to bone, Ti6Al4V alloy, and ultra-high molecular weight polyethylene. Von Mises stresses, minimum principal stresses, and sagittal displacements of the prosthetic condylar head were evaluated under multiple loading conditions. Results: The spherical curvature produced the highest contact stresses and the most restricted condylar translation. Intermediate elliptical radii reduced von Mises and minimum principal stresses by up to 50–60% and increased sagittal translation by approximately 45%. Sagittal curvature geometry showed a greater biomechanical effect than occlusal plane or loading direction variations. Conclusions: Elliptical sagittal curvature radii optimize stress distribution and functional translation in customized TMJ prostheses, providing quantitative design guidance to improve implant performance and longevity.